In Uncharted 3 multiplayer (co-op in particular), do you earn more money per kill/medal/objective on the higher difficulty levels, or is it all the same and you just increase difficulty for the challenge?
Yes. While the Easy and Normal modes offer no cash multiplier, playing on Hard and Crushing gives a x1.5 and x2 multiplier respectively, so for example, a normal kill on Easy and Normal is worth $80, on Hard it's worth $120 and $160 on Crushing. Headshot kill is worth $200 on Easy and Normal and $300 and $400 on Hard and Crushing respectively. Same goes for medal rewards.