Are there a lot of hills in your area? Road bikes with gears are generally more expensive than single-speed bikes, but you may need more than one speed if there a lot of hills where you live.
I bought a single-speed bike from
here. The problem with buying bikes online is that you may buy the wrong size, or you may assemble it wrong. Assembly isn't really that difficult, but some people are intimidated by doing it themselves. You also might need to take your wheels in to a bike shop to have them trued, if you're buying the bike online.
There's nothing wrong with buying a used bike off of Craigslist! That would probably get you the most bang for your buck, providing there are a lot of postings in your area.