There are a few vendors that have on-line pricing so you could submit a file for quote. The cost has some down somewhat and depends mostly on "time in the tank" which is based on part volume (actual material volume, not including air spaces) and desired accuracy. I've bought some "finger sized" parts (through work) that are under $100. Usually you need to provide a CAD model in a 3D format such as STL (a tesselated output from your CAD system) or one of the more common "editable" formats such as Parasolids (i.e., Solidworks, SolidEdge), ACIS (AutoCAD 3D - I think) and I'm sure ProE. Having someone build the 3D CAD model for you would add $. Also, depending on the process used, the material properties vary. The traditional photocured parts are brittle - suitable for casting but not hard use without reinforcing. Some of the newer fused deposition processes using, for ABS for example, might have more utility but maybe less accuracy. If you can make the CAD model, I've found the best candidates are complicated little parts that are hard to make other ways, that don't have super-high precision or strength requirements.