"Similarly, how does it make sense to pay Chamakh the same as Wilshere?"
Of course it doesn't now, but when Chamakh won his contract Wilshere hadn't yet joined the first XI.
There was a huge level of (as it turned out, misplaced) enthusiasm from many about his arrival and that continued for at least half a year or so.
He was supposed to be the guy who could play the roving, linking CF role for us when Van Persie was (inevitably, as we then believed) injured for stretches.
And he was supposed to be our aerial threat, as ably demonstrated by several brilliant goals for Bordeaux the season before he arrived.

Chamakh also arrived on a free, naturally leading to some inflation in his overall package.
You could say many of the same things about Squillaci, who was hailed as a cool head and an experienced general in our defence for quite a while after he came along.
All of which goes to show that a contract can only be judged on the circumstances at the time it was signed. We almost invariably write long contracts for players we want who aren't too old, because we want control of their careers. With the control and the long contracts comes the risk of a serious deterioration in their performance.