Lake Baikal to Moscow

This is depending on what class the ticket is, where you buy the ticket and when and how many of you travel.

I can tell you, the Irkutsk-Moskva "base" ticket is 85 Euro and the 2nd class 4-bed supplement for it is 78 Euro, hence the total is 163 Euro.
But: this price (for the base ticket) is valid in the case if you buy your ticket in Slovakia or Czech Republic only. Otherwise it's 142 Euro (in most other countries), the supplement is always the same.

Keep in mind, traveling from Ulan Bator and buying Ulan Bator-Moscow ticket with Ulan Bator-Irkutsk bed supplement first than adding the Irkutsk-Moscow bed supplement will be less, than the domestic ticket on "better" trains.

In general, forcing international tickets can be a good idea if you plan to use "very good" trains and the better the class, the larger the saving. If you plan to go as cheap as possible, the "Platskartny" ("third-class") domestic ticket in the slowest train is impossible to beat.

Because of the above curious situation, going from China, Mongolia or from "East Europe" (do not want to discuss how justified the terminology "East Europe" is) can be a good idea if you like to travel on "better" trains and in high season.

Since you are from Poland: as bizarre it is - you better to either buy ROUND-TRIP tickets in Poland (AFAIK, one-way is not discounted) from Poland to Irkutsk or ask your friends/relatives to travel to Slovakia or Czech Republic and buy for you the Irkutsk-Moskva 85 Euro "base" ticket and send you to Australia.

Once you have the "base" ticket, the railway agency in Irkutsk will sell you the bed reservation in any train, even the bast ones like train #10 for 78 Euro. If you are lucky and they are in good mood, ask them to sell you the reservation for "СМПС-МПТ", not "СМПС-Восток-Запад" tariff. Than you will pay 40-50 Euro instead of 78, but it's not official.

I suggest reading the New RU Train sticky which gives lots of information about the trains.

If you don't plan to have someone purchase the ticket that LXL2 mentions, you should understand the price of the ticket varies with the train number, the class of berth, whether meals are included, the time of year (seasonal supplements) etc. Thus it is impossible to answer your question.

Ruth

Little addition: going on the "lucky" days with the train #1 and catching the ZC car is very cheap. The car is usually empty, here is the current situation showing all 28 beds being free a mere 3 days before departure from Moscow:

http://img187.imageshack.us/img187/8154/picture1zl6.png