If you're shopping for a new air compressor, consider the ways, and how often, you'll put it to use before buying the cheapest one. Everything depends on how much you'll be running it, and what you plan to use it for. Compressors that cost less than $100 are very portable and will do a fine job of running a trim nailer, filling tires or blowing dust off your clothes—but that's about it. You'll have to be patient; they take a long time to get up to pressure and to fill tires. If you do decide on a cheap compressor, consider it a “throw-away” tool and be prepared to replace it when it dies. Virtually any repair will exceed the replacement cost.
If you can afford to spend about $300, you can get a portable compressor that'll power most DIY air tools and last for a couple of decades (see Photo 2). Look for a compressor with a cast iron cylinder, oil lubrication and air output of at least 4 cu. ft. per minute (cfm). You'll have to change the oil on schedule to keep it humming. But the longer life outweighs the hassle. Also be aware that oil-lubricated compressors inject a fine oil mist into the air line. So you'll need to invest in a separate hose and a filter if you're going to use a paint sprayer.
You can find less expensive, oilless compressors ($129 to $199) that will put out 4 cfm, but don't expect them to last as long. And you'll need to wear hearing protection—they're LOUD!
A compressor rated for 4 CFM will run all of the above tools plus lower CFM tools.
If you're a serious motorhead, you'll have to take a larger leap. If you want to run “air-motor” powered tools like impact wrenches and ratchets, you'll have to get serious with a unit that's capable of at least 5.5 cfm with a sizable air tank. Just forget about running air-powered sanders and sandblasters—those guys require almost 9 cfm. Expect to spend $540 plus for a good one. But the only thing that makes them portable is the wheels. They're heavy and bulky.