Dude. You seem to not understand this.
Paying for a players past is taking a guy like say mangold and making him the highest paid C in the NFL.
Paying for a guys future is saying for the next 4 years there is no way in hell mangold will still be the best C in the NFL.
Paying for their past ignores age, decline, system.
Paying for their future means you take those things into consideration and pay them based on the value they will give you going forward and notcthr value they had before, elsewhere, in their prime.