I would think bringing it up to 15 would be easy enough, but as for the change in costs for coaching it to different levels, that is probably going to be trickier.
OTF hasn't changed, as far as I can tell, for ages. In fact, has it not always been 10?
If so, I think a sudden rise of 50% is maybe a little too much.
Also, if it drops down by 30% at ESA, then it would go to 10.5 if you had it maxed.
Depending whether that rounds up to 11 or down to 10 could mean that to get it back to 15 could cost as much as 15CP.
That's a significant investment. In fact that's half of what some managers purchase for CP during the off-season.
I think a lower increase and have the coaching costs remain the same as they are for now.
At the moment it would cost 6 CP to maintain OTF at 10.
An increase to 12 or 13 would see it cost 8 CP to maintain. Provided that 0.7*12 = 8.4 rounds down to 8 at End of Season Aging. If it rounds up to 9, then it remains 6 CP to maintain at 12.