You can get a decent laptop on EBAY these days for around $300. Any lower than that and you're going to be getting project laptops that you'll have to fix yourself... or they'll have minimum specs.
What you'll probably want (as a basic laptop) is something in this range:
40 Gig hard drive
1-2 Mb RAM
Any processor that's around 1 or 2 Ghz
Something like that (without considering other things like accessories and USB ports and such) should be around $200, maybe slightly more, depending on the manufacturer and accessories. Obviously, you can go higher on those specs if you're looking for a better computer.