Best bet would be to find a $100 laptop on CL and see if you can negotiate down to about $70. Honestly, you can part out most laptops for more than $50. For $50, you would be looking at a single core, <80gb hdd, <1GB ram computer, still on XP. It would have been slow when it was first released.
Recently, I was able to pick up a core2duo, 4gb, 320gb laptop on CL for $80 after negotiation.