Having owned all three systems in question, I'll say that I probably spent about the same amount of money on each, but didn't get as much for the cash from some. The A500 is pretty easy to achieve some basic acceleration on, but 020+ accelerators (needed for more than 8MB fast RAM) are harder to come by than 68000 accelerators, and sidecar expansions are a pain in the ass. The 2000's an absolute monster on a desk, but eminently expandable, with a wide variety of cards available cheaply, and it's much easier to get an 020 or 030 board that will take a lot of RAM. The 3000 has some major advantages (up to 16MB onboard and an 030 and flicker-fixer built right in,) and has the best accelerators out of the three, but they're harder to come by and the case is an absolute bitch to work inside (why oh why did people think sub-desktops were a good idea?)
I'd say, if you don't mind hefting a 50lb. tank onto your desk and don't need the sheer horsepower a tricked-out 3000 can offer, go with the 2000.
