Depends on what you plan to do and what you expect. I have two nice bikes, a Rivendell Atlantis and Rambouillet, and I love them-- but my mid-80s Trek, which I bought used 10 years ago for $150, is still fun to ride and still works great.
I'm not particularly an aluminum fan (it's fine, I just like steel better), and REALLY not a fan of low-end or used carbon forks, because you don't know what kind of treatment they've had. Older Peugeots and Bianchis can be fine or junk, depending on what model they are and how they've been treated. My Trek was a terrific buy, and i bought my wife's commuter, a barely-used Bridgestone MB3, for $25 at a yard sale, but I'm a decent mechanic and could spot potential problems.
With a tight budget, I'd look in thrift shops first, but at least around here, you see 20 mountain bikes for every road bike, and most of them are trash. Don't look for a specific brand or components, but for a decent bike that fits you. Craigslist can be great if you get lucky, but don't be tempted into buying something that ALMOST fits, because it won't. You can expect to need tires, maybe brake pads, and eventually a tuneup, which you can learn to do yourself.
It IS possible to find good bikes cheap. My Atlantis and Rambo would cost probably $6500 to replace (I didn't pay nearly that years ago), and while they're definitely better than the Trek, they aren't $6350 better.