I would recommend the RigidBot. The standard sized one is 10x10x10, they come at 800 assembled or 579 as a kit. I think they are nice because they are extremly modular. If you want to go bigger down the road it is simply done. http://rigidbot.com/product/rigidbot-regular-assembled/

Otherwise if you wanted to wait a few months the Genesis 3D printer by X3D is a great modular printer as well. They will be sold at about 350. http://www.x3dmachines.com/