e.g. have training costs go up per skill level in a non-linear way. The number of uses required to increase skills goes up geometrically, so I figure training costs should too!

I believe this should go into the "construction set" area.
anyways, I am not entirely sure, I would just say play around in the CS a little, see if you can locate what allows training...but this wasn't to tell you what to do, just a friendly bump

I am pretty sure that this is impossible without a scripted, OBSE-dependant mod, but the mod needs only be a few lines.

I might consider adding it to Enhanced Economy...