DIY 3D printers will probably get to be extremely cheap, as more and more parts of the printers will eventually be able to be printed at home. However, I don't see prices of already built 3D printers by companies like Makerbot, Cubify, and perhaps HP dropping below $500 anytime soon. Maybe in 10 - 15 years we will start seeing sub $500 mainstream 3d printers. That's when there will be just as many 3D printers sold as 2D printers.