In the first levels you dont need many tokens to have a strong team.
The players of U21 can be good, just dont look to there token price, but for there market value.
Last season i received youngsters 2 times in the first i received a 15T players with the highest price you can find for a player with that Q, i sign him, after 16 days i received onether one, this time a 24T player, but His price was mutch lower that the first i received for 15T.
I then understand that the token asked are random.
This season again a received a 15T youngster 2SP away of 4* and His training was above average (40/80/120), he is now a 5* players with only a few greens spended.
With this i say that it can be a good option sign them, just make the question how many tokens Will cost you a 18 years old player fast trainer in the first week of the season, and you Will see that 15T can be cheap.