Cheapest, best ground transport Bankok-to Ulaanbaatar

No budget flights this route, I think!?!?

As far as I know, you are right.

I'm not an expert in this area of the world and therefore, I can't recommend you an overland route but Did you think that you'll need a double entry visa for China if you want to do it overland?

How long is your trip? Will you be returning to Cambodia or head for somewhere else after Mongolia? What's your passport? Is there a preference for going through Laos or Vietnam?

Completely overland is doable with train as primary method inside China and Mongolia, and there are a few route options, but it will take a long time, and for sanity's sake you'll probably need/want to stop off at least 2 or 3 times and get a night in a hostel bed. Even using overnight sleeper trains. Other options involving (partial) air travel:

1) PNH-KUL-TSN (Tianjin) on Air Asia then train/bus to Beijing then train or bus to border then Mongolian local train to Ulaanbaatar. Due to connection times, you'd have to stay in KUL for balance of day and overnight. Probably about USD 250-ish for these flights if you book this far in advance. About another $40 total for overland transport modes up to TSN-Beijing-Mongolia border, excluding any overnight accomm in Beijing.

2) Overland via Laos to Kunming, China. Flight KMG-PEK then train or bus to border then local train to UB. Flight KMG-PEK, if purchased 3-4 weeks in advance, can be had for about USD 150.

There are lots of permutations and routings, but those two are probably the best bets for halfway reasonably-priced flights to speed things up. There are a number of threads/posts from earlier this year on doing the Beijing-Erlian (border) part and vice-versa on the cheap, also some posts on the local Mongolian train Zamyn Uud-UB.

Thanks for the info:
2
+How long is your trip? Will you be returning to Cambodia or head for somewhere else after Mongolia? What's your passport? Is there a preference for going through Laos or Vietnam?**+

I have much more time than money. Scouting Mongolia for possible next residence, miss seasons in Cambodia where I have lived for 4+ years/ probably stay 2 mo or so but flexible/ on (unfortunately) US passport so $140 for double entry China?/ I have traveled both but VN has train system the length of the country, Laos buses are getting better/ so, 50-50 visas similar in price but Laos on arrival/

Which airline for China hop to BJ? Kayak best for price? On posts you mentioned OP said $65 possible for land travel BJ-UB? Agree? So Cheapest from Laos/VN border to BJ would be (ballpark)?

Thanks for all your help!
Drifter

The US passport is not so bad in this case. Yes, for China visa it is $140 whether for single, double, or multiple entry, but for Mongolia, US citizens get in free for stays up to 90 days, which should be adequate for a scouting mission. If you are resident in Cambodia, go for the double-entry Chinese visa, definitely, and ask for 60 days duration of stay per entry to give you greater time options, even if you end up not needing this much. I do not think you will be able to get a 12-month multiple entry visa in Cambodia--which is standard issue to US citizens if applying in the USA.

From Bejiing to the Mongolian border, you have 3 options, each will cost you about US $30-33 excluding food on the journey and any accommodation or costs you expend if you stay in Beijing (which you probably should plan on).
1) K3 or K23 train Beijing-Erlian. Timing of train requires an overnight in Erlian. Downside: only runs on certain days and ticket purchase requires very specific procedure, best documented in this thread , which you should read and thoroughly digest. Approx $23 train and $10 night in Erlian = $33
2) K573 train Beijing-Jining South (Jiningnan) overnight, then switch to public bus to Erlian. This has the advantage of being a daily and purchaseable at any ticket office without special procedures. Timing is 20:45 -05:00 into Jining Nan. There are other trains doing this route but the K573 has the best timing for what you need. Not sure timing of earliest bus Jining-Erlian but probably about 07:00 am, which would put you in Erlian around 12:00 noon. Hard sleeper US $20 + bus $11 = $31
3) Sleeper bus Beijing-Erlian. Timing is usually about 17:00-05:00. Have to kill a bit of time in Erlian before border opens. Advantage is easier to get a bus ticket a day in advance than a sleeper train ticket. Cost about $30.

Add to that: $7-10 to get from Erlian to border then all the border formalities and into Zamyn-Uud. About $27 hard sleeper Z-U to UB. So yes, BJ-UB can be done for about US$65 plus food.

The biggest determinant of your travel cost and time is going to be how you get across China. For current details and pricing on overland through Laos, ask on the SE Asia board. I believe all things considered, going through Laos and heading for Kunming will be cheaper than going through Vietnam. Do not use kayak or any other western site for China domestic airfares. Try www.ctrip.com or www.elong.net or www.travelzen.com and remember that domestic airfares in China are often most heavily discounted 2-6 weeks out, not months in advance which may show only full fares. Doesn't really matter which domestic airline you use, but go for a nonstop.

Having problem determining train fare from Nanning to Beiging. Man in seat 61 says $71 but notes extreme fair increase in 2009. What is good site for train prices? For bus fares and prices?

Drifter

Best sites are official but Chinese language only. Try this one which has an easy train search function and which keeps up-to-date on accurate schedules and pricing.
www.travelchinaguide.com
If you hit the "select it" at the right of a particular train, the detail on the various types of seats/sleepers for that train, with pricing, will show. It does not reflect availability (or lack thereof). Note that this site is a travel agent that can arrange train tickets for a fee, but I am only vouching for the search function so you can see official prices that you would be charged if you were buying yourself at train station I have no idea about their ticketing services. Focus on the RMB pricing. Seat61 is a great site but sometimes is behind on the pricing and cannot provide the detail you need. Another site is www.cnvol.com which is also pretty accurate, but the relevant train listing is arranged in a more chaotic fashion so it takes a little longer to work with.

Remember that train tickets in China are not hop-on/hop-off. If you plan to get off for sightseeing, rest, etc between Nanning and Beijing, you should buy only immediate next segment you are traveling out of that station. Sleepers can be very difficult to get on short notice, yet the buying period is only up to 10 days in advance for the average train.

The only way you will get bus fares is to do internet search on travel forums for some relatively current info on the journey you want to make. Schedules are highly localized and subject to change, and rarely are bought more than a day in advance (and that mostly for sleeper buses). Long distance bus travel should be broken up in chunks, I suggest not more than 6-7 hours between stopover points where you get a hostel bed. Sleeper buses are not that great a way to travel, definitely not if you are tall, and there are safety concerns.

The train from Hanoi to Biejing W/ Stops every 12 hours or so seems best for me! When I stop, I like it to be 2 days. Gives me time to wash clothes, shave my taint, ETC

The website you gave me for china trains is very good. Best extrapolation, Price of train $150-160 soft sleeper/ about 2/3 that hard sleeper and $50 or so soft seat. Seems better price if you use local train and/or bus From Hanoi-Nanning and then Train.

Anyone like the soft seat option? I did one 12 hours in Thailand and was OK and I have long legs, so that might be an option if I go in 12 hour stints!

Would Kunming-Biejing be better for scenery, less people and more agrarian society?

Bus goes from LP, Laos to Kunming direct but seems tad expensive! Knowledgeable ex-pats on forum like to make alt trans to border and bus from there to Kunming
Most International bus runs are quite more expensive and only few actually cross the border, so locals

much better over borders, generally!

Drifter

Checked hostelbookers and China prices for hostel beds about $5 dorm but probably better quality than in Laos/Camodia!

Drifter