Really not much. I mean they are so cheap nowadays, I saw a nice one this morning for $369 core 2 duo. I even saw an i3 with WLED screen for $430.
Once they get a few years old, the batteries go and then other things start going like touchpads/mice, charging jacks, screen, and hdd. Once the warranty is up and something goes, your poop outta luck.
I wouldn't pay more than $100 for it and that's "IF" I was desperately looking for a used laptop.