Here's one way to mitigate mental buff abuse:
The first - I dunno, either 5 or 10, let's go with 10 - the first 10 points of buffing cost the standard amount. So, buffing int by 10 would be 14 + 12, or 26. The next ten points of buffing cost double. So, buffing int by 20 would be 14 (the first...