I noticed some time back that the cost to train characters sometimes varies without any apparent reason. I just got an amazing example of that.In Stone City, with a level 1 party, it cost 20 to train my male human knight to level 2, while it cost 41 (!) to train my female human ranger. Training from 9 to 10 cost 187 and 374 respectively. (Both characters had merchant skill at level 1.)
I have no idea what prompts variations like this, but they often happen. This was the most extreme one I have seen.