I like the idea behind the F3 suggestion, above. Here's an even cheaper Nikon option that will get you shooting some great pictures: the little-loved FG.

You can get one for - no kidding - $30. Also, you could pair that with the 24mm f2.8 lens - a hundred bucks will get you a good one - and throw a ten-dollar filter on the front and you're DONE. Less than $150, and you have an AWESOME rig that you can upgrade easily later on! That leaves you $350 left over, which will buy you somewhere around 81 rolls of 36-exposure Tri-X 400, which is a minimum of 2916 exposures. Even I could get a good one with that many frames...