i intend to go F&E to korea too! hopefully can find a friend to go with me or else i'll just go on my own.
i've been reading up and adding up some expenses and i realised that for korea, it's cheaper to go for packaged tours then extend if you want more time as compared to going f&e. the air tix is almost the same price as a tour package (but if you buy from an agent it'll be cheaper but not much i guess). meals are about ~S$10 from what i've read and language is definitely a barrier so if you don't know how to read hangul or speak korean, package might be a better choice.
but for me, i find going on packaged tours a bit rush and the attractions aren't my cup of tea esp when they bring you to those ginseng factory or those wholesale centres to buy pearl powder.

besides, i've read that though some package tours include attractions like lotte world and everland, it's quite a waste of time cos the place is so huge you won't be able to finish it in a day. but... if you die die must go jeju island, i think it's cheaper if you sign up for a package tour. oh yah, accomodation is quite expensive in korea so you might consider homestay-ing. about ~USD$25 and above for a night.
the prices of stuff in korea is as expensive as those in japan.