Getting to Incheon Airport - desperate

Breakdown of air ticket quotation between Tokyo and Seoul on Korean Air after taxes and charges:
http://farm4.static.flickr.com/3097/2919731345_1232dc2ec8_o.jpg

I presume a one way ticket may cost more, though I haven't verified it.

How to get to Beetle departure point in Fukuoka:
http://www.jrbeetle.co.jp/english/access/index.html

You can do Fukuoka-Incheon in one day if you push it, but this depends on when you flight is.

The cheapest option would be to get the slow overnight ferry from Osaka/Kobe or Fukuoka (depending on where you finish your Japan trip), the subway to the bus station (W1300, 1 hr) and then you can get a direct bus to Incheon airport (around W40,000, 4 hrs) or Seoul (about the same).

If you do the Beetle-KTX route (faster), I think you can get a special "through ticket" - but this may only apply to the Tokyo-Seoul route and you'd have to ask in Japan. Either way, you'll end up at Seoul Station or Yongsan, from where the airport is around an hour away by bus (around W10,000) or a bit longer by subway. The train station is around 10-15 minutes walk from the international ferry terminal, or you can take the subway.

If you stay overnight, it'll be cheaper in Seoul that at Incheon - you should be able to get a motel or hostel in the W20,000 range.

There's also a shuttle bus between the international ferry terminal and the train station in Busan, but I'm not sure how often it runs. A taxi to the train station shouldn't be more than 3,000 won if you don't want to wait or to walk.

my flight out is at night time around 8pm....

The Beetle plus KTX is a good option, but it depends on price, as I have recently found out that a return flight from Fukuoka to Seoul is the same price as getting there by way of the Beetle & KTX. However I think a single flight will be comparatively more expensive.

If time is on your side then the ferry/train can be interesting - you could spend a couple of hours in Busan before taking the train to Seoul & would then have much of the next day in Seoul to explore before getting your evening flight. It is probably a little over a mile from the Busan ferry port to the train station, but if you have bags then a taxi is easier, or walk to the nearest metro station & I think it is just one stop to the station. If you decided to have a look round Busan whilst there I think that the station has luggage lockers.

There is also a cheaper ferry(9000 yen) between Fukuoka and Busan, but it takes six hours instead of 3 hours with the beetle(13000 yen). Google for "camellia line fukuoka busan" and you should find the website, but only in japanese and korean. You can also go to Busan via Shimonoseki instead of Fukuoka. KTX from Busan to Seoul takes three hours and there are a lot of them.