Don't expect to save money by buying a compressor, no matter how cheap you get it, the simple fact is maintenance, filtration, oil, etc. for a small compressor costs a significant fraction of what your spending now on air fills. Don't expect to save much time either at 20-30 minutes per tank fill with a small 3-4 cfm compressor, it would be quicker to buy 10 tanks and drive the hour each way to get them filled. What small compressor ownership does give you is flexibility, you choose when the time is spent filling tanks, your not locked into a diveshops 9-5 Mon-Fri hours, or rushing there after making a dive on Saturday morning to get tanks filled for Sunday. A compressor can also get you a lot of diving friends you never know you had, which leads to free air fills, liability concerns, sure they would not sue you, but what about that next of kin if something happens. Once you have thought through all this and have decided to go ahead and buy a compressor then you can start thinking about what kind and size you want. Is portability a major concern, there are plenty of units out there in the 3-5 CFM, 100-200 pound range that typically sell new for $4,000 and used for sometimes much less (I bought a 30 year old 3.2 CFM unit a few years ago for about $250, plus driving over a thousand miles round trip to pick up). The problem with most of these small units is they give up a lot for this portability, and are not designed to fill more than a few tanks at a time without need to cool down. You can also find real good deals on much heavier (both weight and durability) units in the 5-10 CFM range these units are intended to run hour after hour often with nice features like automatic drains (instead of manual draining every 10 minutes), better gauges, etc. These units likely weigh in over 500 pounds so moving them is a bit more of a challenge, many have 3 phase electric motors which can to hard to power without expensive phase converters or new motors from a typical home single phase power system, etc. Its just up to you to decide where to go from here. The process of filling the tanks themselves is basicly a simple matter of connecting a hose and turning some knobs compared to compressor selection.
