Of the few sub-$500 3D printers, the PrintrBot Simple appears to be the most sturdy, and you get a lot for your money. The printer comes fully assembled and has a 4-in x 4-in x 4-in build volume.
It prints using PLA only and supports Windows, OS X and Linux. Because it comes with an SD card slot, once a 3D CAD drawing is uploaded to it from your computer, you can detach the computer and go on your way while the printer continues its build.
The printer bed is not heated, which could make it harder to detach objects, but, since it prints using PLA, object warping shouldn’t be a factor.
A PrintrBot simple kit is $299, but requires assembly.