Yipes! I suppose that depends on what it is....I'd max out at probably $200 for a fish. Ummm...maybe the same for corals. It would really depend on the day and how much spare change I have.
Would you buy a fish that looked starved in hopes to revive it with TLC?
If the vendor would feed the fish and if the fish ate well, I would probably buy it or try to jew the vendor down and buy it, thats if it was a fish I really wanted.