Part+the job = $50 max.
But if you still have the warranty - you pay nothing. Just say that the key was acting funny from day one.

I just had like 3-4 keys replaced on my Korg 01/W FD. It only cost me about a $100 canadian (like $70 US).

PS
I don't mean to be kinda basic, but did you mean that you hit the key harder to get aftertouch going?
'Cause to get it going all you have to do is press the key gently after it's already down. If you already know that....
I always found Roland aftertouch (except the D50, JD800 and some XP boards) kinda stiff. You have to practically stand on the keyboard to get the aftertouch happening.

Good luck & keep playing!
-ED-