Oftentimes, just a couple of pieces are responsible for the high price, because they're rare in the specific color being used in the MOC. Sort the part list by price, and you'll quickly be able to identify what the problem is, and see if you have similar pieces already that'll do the job, or if you can just substitute a different color that's cheaper to get, for example. It also helps to have your own collection logged on rebrickable, as it'll show you how many of the required pieces you already own!