You are going to be in tough shape at that price. The price for the bare drives is $220. Add decent raid and ethernet and you are out of range.
Not what you asked for but some of the newer external drives are raid1. Here is the WD at $350:
http://www.newegg.com/Product/Produc...82E16822136100
If you have an old box around the cheapest/best way out would be to through the drives in it. I think there is a Linux distro out specifically for NAS (free nas?). Toms did a review of solutions a while back and this was the best answer.