Just came back from my Jeju, YongPyong & Seoul holiday. Best family holiday you had, especially enjoy our 2 full days ski at Yongpyong.
If you planning to go on your own, here is a contact you can call for cheaper ski wear/equipment rental, Ski lesson (taught by a english speaking Australian student there), and cheaper ski lift .....
You can contact Choi Yong Gil, 010 88131976 or email him at 1976Gil@gmail.com
They will drive will to theri shop to select your equipment. if you have a group, remember to put your neogotiation skill to test.
You will definatelly save 40 to 50% comparing to getting it direct from YongPyong resort itself.
If you need transport from Airport to YongPyong, I have taxi conatct too.