We'll be up front here: this project isn't easy. However, if you're up for a challenge that will probably take up several weekends, these plans to build your own 3D printer for under $500 are pretty killer.
Instructables user AlmusYang has put together a thorough guide to building a home-made 3D printer. The process isn't simple, of course. You'll have to order laser-cut parts and other mechanical pieces that you might not be able to find at your local hardware store. However, since even low-quality 3D printers are still pretty expensive, this may be worth a shot.
Like our own guide to building a kickass robot arm, this isn't going to be something you accomplish in an afternoon. It's best to view this as a learning experience and a long-term project. If you can make it through, though, you'll have a pretty killer tool.
Building a 3D Printer Under 299$US [Instructables]