A players value can go up or down during the year he is under control. Some have signed for millions, others have signed for practically nothing. Some do not even get an offer.
It has very little to do with how high you were drafted or how low. It has much to do with where you're likely to be drafted if you do not sign with the club that has control.
The negotiating leverage is still present. The player does not have to sign. He can choose to re-enter the draft or continue in college.