The bed plate needs to be flat, rigid, and thermally conductive. If you can get copper that is all those, go for it. However, in many printers the bed moves in the Y axis, so having lower mass would be better, too. Machining copper can be quite difficult. As an alternative, cast aluminum is easy to machine, lower in mass, almost as thermally conductive, and readily available for much lower cost than copper. If you have some sort of scientific application that requires even heating to within 0.01C over each cm of the bed, maybe not, but for a 3D printer that prints plastic it is fine.
Here's a thermal image of a cast tooling plate bed on a Taz printer. Note the temperature variation (or lack thereof):
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]