Definitely plenty of used bikes on craigslist. I recently picked up my 1st road bike for $600. Excellent condition 2011 Scott Speedster S20 with full 105. I made sure that it was the right fit/size before I bought, however. I've put about 125 miles on the bike already with less than 2 weeks since I bought it. You don't need to spend a ton to enjoy riding.

You can also check bicyclebluebook.com to check approximate value (not sure how accurate though).