If you just get air fills, it will be hard to justify economically. Convenience is another matter but if you're within 30 minutes of a shop you will probably do better using the shop if its hours work for you at all. Factor air tests into your costs, and strongly consider a CO analyzer (with its test gas costs). Actually, consider that whether you buy your own compressor or not.
For me, local Trimix fills were $250 and I use a fair amount of Trimix, Nitrox, and O2, the latter two for decompression gas. A T bottle of He is about what one fill costs locally, cutting Trimix costs by 75% or more for most depths. At that rate, I paid off the compressor, bank, gauges, analyzer, and Haskel in about 3 years. But I was fairly lucky in finding a lot of used stuff I could use at reasonable prices, too.
Owning a fill station is, frankly, a bit of a pain. I do one air test a year for OCA with old filters, and I run fill-size towers. When I get it back and it's passed, which it has every year so far, I change the filters. That's about $250 a year in tests and filters, another $75 a year or so in oil (synthetics are best but cost more) and once in a while repairs (like a new 4th stage head and new 4th stage coil) and upgrades. Then there's keeping the bank hydros current (if you choose to), and maintenance on the bank valves and those on the whips, and so on. So direct costs for me are about $350 a year plus repairs.
Startup costs include the compressor, the fittings and hoses or tubing to connect them, new oil, and new filters, at a minimum. Having bank tanks beats not having them by a wide margin but mostly only if you have a continuous duty compressor.
Also alll your diving friends will want fills. I don't have insurance for that, which makes me *extremely* selective about to whom I provide fills. On the plus side, my compressor should run more hours a year than it does. On the down side is liability. I am pretty confident that those to whom I provide fills understand and accept the risk, and that their families do as well.
On the balance, I am very happy to have my own fill/blending station, and I would not have it any other way. I have more training than many LDS's and I know damn well who is breathing the mixes, which makes me very, very cautious. However, from an economic standpoint, I don't plan to test 8X a year, either (4X per year before and after filter changes), which is the gold standard used by those servicing breathing air systems locally for fire companies, nor 4X a year after filter changes as many local LDS's do. So I do accept some risk, too.
Hope these meandering thoughts are helpful...
