First of all, Westbrook would have been risking injury and possible poor play (remember there were plenty of ?s about him shooting too much and inefficiency while not being true PG) to earn that extra money. And as someone else said, he would have also had to make the All NBA Team which was a challenge cause he has never made it before.
And who cares how Collison got the money?? The bottom line is that Collison got the money, even if its through signing bonus. I'm sure if OKC had offered Harden a deal in which he gets $60 million, even if its through signing bonus (I don't even think this is possible) then he would have taken it.
Ibaka was in a situation where there's only a handful of teams that he would have been effective on and that can truly show his skills. OKC was the best atmosphere for him because he could just block shots, play defense and be on an elite team while getting the fame for being OKC's "defense guy".