If he plays in a position you need or it is dificult to find in the market buy it.
The token price dont mean they are fast trainers, i got 2 really fast trainers for 15t and a medium trainer for 24T.
See His price before you sign him, to do that, just compare him to another player of you team.
It also depends If he is a low 4* or a high 4* near the 5th star.
If you dont need the position and the price dont seems the one of a fast trainer, dont sign, in 5 days, he Will lose a star a be one year older.