Ennis may spend his money freely once he has any but he also seems quite touchy about being regarded as "poor". For instance, when Jack offers him a loan when they're up on Brokeback, his reply is quite heated as though Jack had somehow offended him: "I don't need your money, I ain't in the poorhouse". The money he's earned on Brokeback that summer willl help but I'm not sure he's that far from the "poorhouse" when he marries Alma.
I think maybe we're supposed to see him as more fiscally responsible once he is married and has children. At least, as Southend points out, the reason he gives for not moving into town is an economic one:"...rents in town's too high". Alma,of course, wins that argument, however economically correct Ennis might have been.
Money has its role to play as well in FC. As Ennis tells Jack: "You forget how it is bein' broke all the time".
Overall, the question of money and maybe social position form an interesting backdrop in Brokeback Mountain. If the "expensive gun" scene had made it into the final cut, then I think this would be more prominent in any discussion of the movie. Nevertheless, the subject of money is still there.
