I very often use cast aluminum tooling plate in my setups. It is already machined (to a 25 RMS finish, whatever that means) and with a good thickness tolerance. It's very stable when machined, so it can be clamped to the mill table and used sacrificially, or used to make fixtures. I also make engine parts from it.
I use it because I had stumbled across a surplus source of it. A company uses it in a a manufacturing process, and has tons (literally) of stock disks of it on hand.
All the stock is round disks which have a few small holes drilled near the periphery. The prices per square inch is considerably lower (say 70% lower) than you would pay for the material elsewhere. I think it is ideal for hobby machining. Those in the U.S. may want to check it out here:
http://www.sandsmachine.com/alumweb.htmI'm not affiliated in any way, just a satisfied customer.
--Tim