Mine was given to me, doubt if I could afford to buy it. Its actually cast iron, about 1/2 thick with webbing and cast in sockets for the legs. About 30 x 36 (I could measure it) a heck of a lift for two people. Its mounted on steel castors that actually swivel and roll as there supposed to do. Being cast iron weld spatter doesn't seem to stick, a rub with the sanding disk from time to time keeps it in pristine. Only drawbacks are you can't tack parts to it and the webbing makes it awkward to clamp along the sides.
My previous bench had grating for the top. Made it out of catwalk grating, 1 x 1/8 steel bars on edge about an inch apart supported at the end and middle with angle iron. I liked the design, dirt fell through so it never needed wiped, you could tack to it, clamp anywhere on it by fishing the welding clamps through the spaces and it was strong. Would still be using it if the current one hadn't come along. It will go back in service in the blacksmith shop.
How thick? I'd go at least 1/2 as Mayhem did if you go plate but would probably go with grating again if I needed to build one. The grating is probably a lot cheaper than 1/2 plate.