Depends on what you are going to use it for. There are various factors into this. So fill out this http://www.tomshardware.com/forum/50797-35-laptop-buyers
and we go from there.
As someone like me for anything under $400 my best choice was an ARM based samsung chromebook which I am typing this post on right now which cost $250. But it obviously wouldn't be a good choice for a lot of people if they ever wanted to do anything more than basic internet browsing and typing up papers in google docs.
So more information is required so we can better help you. And because I feel like it a car analogy because I'm bored. You ask me what is the best car for under $20k I tell you Prius C One, you buy it but your needs are more suited for a pickup as you haul stuff. So now you have something that is practically useless for you, but you have great MPG. But you still can't haul stuff which is what you needed the car for.