For my experience with academy players, and i had a team with 22 players from academy.

The price in tokens isent the only factor of training speed. It also depends of quality, the token player can be between 3* and 4*.
So you can get a 18T fast trainer that a 24T

The price can be 12T, 15T, 18T, 21T, 24T
I posted here this, with photos, maybe a year ago.

This season i got a 4* for 15T and is training speed is good.