Low end- You could probably find a Karaoke system for pretty cheap at places like Sam's, Wal-Mart, or Target. It'll have mixing capabilities mixed in. Only downside is the lack of the wireless mic, which you could get for anywhere from around $40 to $300, depending on what you're wanting. (Check out buy.com and amazon.com for the wireless mics.)
Now for a little bit more, you could buy the same wireless kit above and throw in a guitar amp ($30-$50, depending on ampage and size), and get a cheapo mixer by hitting a musical instrument consignment place or trying Best Buy or Radio Shack. Remember that when it comes to small venues (like a 20' x 20' room), 7-10 watts of amp can be plenty. (My daughter has a 4-watt guitar amp, and that sucker can get plenty loud- even on battery power. Plus, it was only $22 and it's extremely portable. Got it on Amazon.)
Or... Replace the guitar amp with a dolby surround system. You can buy a 300-watt system now for around $100 bucks. (I got mine at Kmart- and it had the DVD player built in). You'll still need the wireless and mixer, of course, as well as a couple of 100' spools of speaker wire to run for larger venues. The cool thing is that, if set up right, you can control your sound better, and premix cool audio effects to use. (And you can use it at home, too!)
Or, replace the mixer in either the amp or surround setup with your laptop. Use the mic in for the wireless (or just a regular mic), your CD drive for your music, the volume control on the sound card as a mixer, and output it from your headphone or line out jack to connect to the speaker system. (You'll have to hit Radio Shack for the right cabling and adapters, but what you'd need is easy to find.)
Finally, there's the real PA system. These have gone down considerably in the past decade. I found a couple of systems for a pretty decent price. Granted, you're still looking at around $250 to $300, though, and it doesn't include a mixer.
This site offers PA systems of all sizes and options, from this really cool 7-watt hip system to the larger professional systems:
Radios4You.