The FA market is an interesting thing. Players don't get paid simply on their worth compared to others at the same position. Their pay is also based on the need of the team signing the player and the number of good players available at the position.
Furthermore, you really can't compare the contract of a player who is 2-3 years into a contract to one signing a contract today. As player salaries tend to raise every year, a "lesser" player might make as much as a premier player who is under contract...but not as much as if that premier player was signing a brand new contract.
Whitner is not a premier safety. At least he hasn't been for the Bills for various reasons (I would argue some of those reasons were out of his control...but whatever). But if he were to play on the Raven's D, I think you would see a much different player. Remember the difference in Jim Leonhard's game after going to the Ravens (and now the Jets)?
I think there will be teams who believe if Whitner is given the opportunity to play with abandon, he will thrive. I almost guarantee he will be paid more by another team than by the Bills. For years Whitner was indispensable to the Bills because of his versatility. That same versatility has probably hurt his stats and his reputation among Bills fans. But I'm not sure that versatility is as important to the Bills defense anymore. They have a fairly credible defensive backfield with some depth.
The Bills should offer Whitner a contract well below what a top safety would command at this time, but above the "average" starting safety, IMO. He probably make something closer to elite pay somewhere else.