That is kind of what I was thinking.For $3000 it should be currently running and have some life left in it. Not new, obviously, but you should not expect to need to do work on it for 6-12 months and it should be in decent physical shape.
For $1500 it should run, and you should be told what to expect to break down soon. (IE the breaks are going out, or it has trouble starting sometimes)
For $500 ... when it breaks it might not be repairable. My last $500 car had a big crack across the windshield, no heat, and threw a rod 6 months later.