Yep, scour craigslist for a solid, rigid, mountain bike in your price range. I've got an old (~1997) Specialized Rockhopper that's now my trails/hybrid, and it's a great bike. You can find these for $100 - $200 in decent shape. My wife's old Trek 820 from 1993 is fine as well. Yes, avoid suspension like the plague!
I'd find the best $100-$120 rigid, name-brand MTB I could find, then spend the money left over (from a $200 budget) to get some new, slick, 26" x 1.25" or 1.5" tires that will accommodate 100 psi. With this setup, you'll be 90% as fast as a real road bike. (I've got over 3000 miles on mine, and compared to my road bike, it's not any slower over most routes.)