Buying potential wonder kids

I use GS 17g to search potential word class youngsters around European Leagues. I find a number of players and try to buy them - GS suggests approx £50k to sign them but clubs continually decline often trying to settle for deals in excess of £30m and on some occasions over £100m. Any reason as to why there are continually these massive differences. I don't want to blow my entire transfer budget on a youth player how will take years to develop. You used to be able to buy numerous youngster every season to slowly build up your squad over a number of seasons.