Hi riminicat,
Your title says you want a good bike, but your post says you want a cheap bike. These are usually two entirely different categories of bike, with very little overlap. If you want to luck out by getting a good bike for a cheap price, you'll have to become as knowledgeable as you can, and then be patient and persistent in your hunt.
Cruiser layout is OK if that's what you prefer, but do keep in mind that cruiser bikes are designed and configured for operating at 5-10mph, and that's what they are good at. You should consider having two strong brakes at the least, since even a gimpy motorized bike will typically exceed 20mph. Multiple gears and a wind-cheating rider position are good things to have above 20mph, and cruisers don't usually have those either.
Try to stay away from department store bikes, new or used. They are poorly made and poorly assembled. If you only have $100 to spend on a bike, try to bag a reputable bike at a garage sale or thrift store for less than $50, and use the rest of your budget to get a professional tuneup.
Chalo