I have an He 3D Prusa I3 kit ($238) which is clear Acrylic. The aluminum bed that came with it would not get hot enough so I replaced it with the RepRap heated bed. Printing PLA was no problem, but when I started printing ABS with higher temperatures, my bed support (made of Acrylic) started to bow. I turned it over and added a sheet of Dow Blue insulating Foam I had from the days when I used to make RC airplanes with it. It is no longer sold, but there is some pink stuff that replaced it that should work as well.
So far this is working out well as it keeps the high temps away from the Acrylic.
I should note that center support in the kit needed to have 6mm added to the bottom in order to support the frame properly. With that added, the unit is rock solid and I have non of the issues that some of those here have posted.
