We just bought my MIL a Google Nexus for $199. It's awesome.
There are now more and more cheap little tablets coming out, but the quality is spotty. Kohl's has one for $79 this week. It's from Polaroid. DH read reviews on it and they were hit and miss-some good, some bad.
I would definitely look around (if you have a Fry's, check there) and read reviews before making a purchase, especially since a child will be using it and won't likely be as gentle as an adult might be.
We did find parental controls apps for the Android tablets, in case that's a concern.