There are tons of nice, older bikes that sell on ebay for less than $200 shipped.
For instance, I'm absolutely kicking myself for not bidding more on this bike.
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWA:IT&ih=002
I would never waste my money on a throwaway walmart bike. Besides, buying a bike that you're planning on upgrading is almost always a poor financial move.