It'd depend. A 64x64 sprite, absent anything else, would cost $32. A very simply-drawn sprite, or a collection of similarly-designed tiles that come out to a 64x64 square, will be less, however.
Hey. I've done some isometric graphics work in the past and would be interested in doing some work for this. My portfolio's [here]. My Pixelation porfolio thread [which contains contact information and the like] is [here].