Just to expand on that, here is an analogy.
Lets use Mike Vrabel.
Mike Vrabel came to NE for the price of what he could be as a Steeler.
He became a better player as a Patriot. And I think he would be a worse player on any other team.
That means we paid the price of how good he was as a Steeler, and got a bargain.
If he were a FA whoever signed him would have to pay for his ability or value as a Patriot, and they would be overpaying, because he wont be that good there.
This could have a lot to do with a tendancy of BB not signing players WE think have great value, but when they go to another team they seem to have less value, and were overpaid.
Vrable is a 5 in Pitt, an 8 in NE.
We sign an 8 player at the cost of a 5.
If he would now be a 6.5 if he left NE and went to another team, that team would pay for an 8.
We would instead choose to sign someone else 5 that becomes our 8. We get 'another Vrabel' that we pay like someone who isnt supposed to be Vrabel.