The money US gave to Pakistan was peanuts. It was like $3b/year, most of which went to facilitating the supply & logistics of and rendering services to NATO. The costs incurred, on the other hand, were staggering.
The common view at the time was that if Pakistan did not support NATO's invasion, then NATO would force it's way through Pakistan, possibly via India, and wreck Pakistan (along with Afghanistan) in the process. I found this view suspect because 1) NATO did not ask Pakistan for support, rather Pakistan itself offered it first, and; 2) if NATO were intent on forcing their way through, they'd have a much easier time going through a very weak Iran instead of a nuclear-armed Pakistan which does not conform to any "first-use" policy on nuclear weapons.