Token price do not reflect performance and/or faster training capacity from my experience.

What I do in these kind of situations is that I add more than 1 player in the same position, and I check to see which one trains faster with the coaches when the training starts on day 8. Say you add the 56.9 and 57.4 players. On day 8, lok how much they can improve with, say, Head coach. If 56.9 will gain 35.2 and the 57.4 will gain 33.6, then 56.9 is a faster trainer than the other one.

Note that this comparison works best when the initial potential qualies are close. If you compare 60.7 and 56.9 players, lower quality one will definitely improve more (unless he is a terribly slow trainer, and in that case you definitely know not to train him).