So why not pay him peanuts?
His agent negotiates that fee on the behalf of his client in the pursuit of reaching the maximum amount agreed to by the club. So yes his wages are partially set by him. There is absolutely nothing stopping him from turning down the offer. His performances also set the bar for his wage. Probably the reason it seems, his contract negotiations fell through. Neither party could agree to a deal.
People degrade Wayne Bridge for doing exactly what Berbatov is doing. Sitting on the bench collecting a large pay check with an occasional game, even though he has a very slim chance of being first choice. Berbatov, like Bridge 'although to better clubs' has the ability to be first choice. Why is he still here battling for 4th?. To be fair though, unlike Bridge I think Berbatov does it for the prestige of being a United player, not because he couldn't give two fecks and is happy just training.
Oh and how is Berbatov not as much of a failure as Veron? Both were signed to strengthen the first team and neither did. Berbatov has had one and a half more years than Seba, but given the same amount of time, I'd say Veron would have been just as effective 'or ineffective'. Berba's purple patch last season papered over what has been a highly inconsistent time at United. Like Veron, good one minute but far to inconsistent to stamp their mark on the first team.