While 25M is a fair price, IMO, we shouldn't take it. David is essential to this team, he's perfectly integrated, everyone knows exactly what kind of balls you have to provide him and that guarantees us 15+ league goals from this man alone. Heck, it's more like 20 goals. I just can't think of a single striker who'd be able to net so much goals for us. Certainly not one we could buy for 25M.
He's 31, so what? does that affect his play? he never relied on running speed or extraordinary dribbles, so I don't see what he'd lose with a few years more. His goal instinct (probably the best in the world) will not vanish and as long as he's motivated he'll be the first on the ball in the box for many years to come.
Some say he doesn't contribute to the play besides scoring goals. That is just not true. I guess you have to see him live to understand that (as: in the stadium), this guy runs a lot more than you'd think. He always manages to be in the right position, and most of the time he'll attract more than one defender to follow him, thus leaving more space to the ball artists.
Also, I'm a huge fan of out and out strikers. A good, solid presence in the attack is essential for my liking, as he'll always give an option to give a pass in the direction of the goal. This struck me especially when seeing Barca-ManU in the CL. Messi was working like crazy between tens of Manc defenders, but he never got to find a striker to pass the ball through. So he'd give the ball to an attacking full-back, but this guy couldn't give a cross in because there was no one to head it in. Eto'o was on the left, on the right, got back a little... Same happened on the other side. Rooney was everywhere on the field but in the box.
When you play dominant football (and that's what we do in roughly 30 matches a season) you just need to have a finisher in the box, ready to receive the ball and volley, head or just tap the ball in. And Trezeguet belongs to the bests in the world for that role.