Unfortunately minimigs are quite more expensive than say a used A500. They do offer a lot more than a plain A500, especially using ADF disk images instead of (mostly non-working nowadays) old floppies is a big plus, at least for me.
If you want to make one yourself, I think you're looking at around 150EUR. At least some experience with soldering SMD components is needed, otherwise the expenses could get higher
All of the new minimig boards are over 200EUR, AFAIK. Even used ones on sites like amibay don't sell any cheaper. There is an option of buying an Altera DE1 board, which should be around 140EUR, but it requires an additional board for mouse / joystick connectors, though this board is quite easy and cheap to make (through-hole components).
What is wrong with your A500, maybe you could fix it?