I bought a used Nikon N70 a few years back. It was (and still is) in great condition. I paid just under $500 for it. (Note: Mine is not digital, it's 35mm.) So I would say around $300 if it's 35mm.
Just like there is a mask for the 6x4.5 there is also a msk you can get for the 35mm to avoid exposure on sprocket holes. site that shows how to shoot 35mm with holes being exposed: site where you can buy 35mm kit with mask;
Canon and Nikon 35mm SLR's would be a good choice because you can find not only good used cameras, but good used lenses for them ... the added feature is that if you ever do purchase a DSLR, all the lenses you have will fit their matching DSLR body.