That's a tricky one. Decent air compressors are not cheap. I would consider looking for a good used unit. Check out your local craig's list in town to see if there are any for sale. You may find that someone is cleaning out the garage, having a divorce sale, or is closing up a shop. There are often times lots of good deals there. Another cool tip is the craig's list search tool that allows you to search multiple cities at once - www.craigshelper.com
If you do find a used unit, check with the manufacturer about replacement parts before you buy it to see if it can be serviced or rebuilt.