So I've done both, I bought a steel frame from nashbar, put my used parts on it and a new fork. it cost me around 250 when I was done, later I upgraded the wheels to 700c mountain rims with cross tires, that cost me another 200. you can do it but it takes time and patience to get the right parts.
Ebay is a great option
http://cgi.ebay.com/2007-NEW-ROAD-RA...QQcmdZViewItem
that's new and would be a great starter.
http://cgi.ebay.com/NEW-ALUMINUM-ROA...QQcmdZViewItem
Is another great bike, New as well.
bottom line is it really depends on how much time you want to spend working on building the bike.