Was hoping clownfish would chime in! It's totally worth it if you dive frequently (~100 per year); and as Tobin mentioned, especially so if you dive nitrox or mixed gasses.
I've done this twice. Back in 2008, some friends and I started a dive club on a remote island in Alaska. We spent $6000 on an 8 CFM compressor, threw all the AL80s we could find into a huge pile, and offered unlimited air fills to our members for $120 a year. We initially had around 10 members, and that easily covered the operating costs of the compressor + VIP and hydro for the co-op cylinders. 8 years later, the dive club has closer to two dozen members and is doing quite well.
I moved back to the mainland in 2012 and spent a couple years slumming it with local dive shop fills, then went back to doing it myself. This is my current setup, with an Airetex 45 and all sorts of goodies. It works very well for supporting one diver and the occasional buddy.
I go through about 8000 CF of 32% nitrox a year, in addition to argon, trimix, deco gas, and air for shop tools. I do biannual OCA tests, there's plenty of filtration, and I have inline moisture, CO, helium and oxygen sensors. I also built a pretty snazzy Arduino-controlled blending stick.
I ran the numbers extensively before I bought the compressor, and I've kept diligent track of my costs. Everything included (filters, oil, testing, electricity, O2, maintenance kitty, etc), puts me at $.10 cents per CF for 32% and $.07 for air. Other gasses vary based on supplier cost (and I'm in Alaska, so they're a bit high) - O2 is $.26 per CF, helium is $1.45 per CF, and argon is $.50 per CF. I own 8 supply bottles and don't pay leasing costs.
In the end, fills work out to about half of what the local "tech" shop charges. Most of the costs are spread out over the year, so unless it's a month where I need to swap supply cylinders (especially helium, OUCH!), it's not so bad.
There are other significant benefits that others have mentioned. Stopping by every week for fills took 2 hours (most of it spent in traffic), and it was a chore that I dreaded. I've experienced every fill station/dive shop complaint in the book, but the worst was putting up with the awful customer service on a weekly basis. Access to the shop/dive boat was frequently used as a threat against customers who complained, and there was no hope of improvement.
Getting away from all of that was probably the single best thing I've done for myself, and I have far more diving opportunities now. Taking care of my own gas fills isn't just convenient, I've found it to be liberating and a great stress reliever!
You're right that the costs can be highly variable, but you could get up and running for $4k + $500 per year in operating costs. For that, you'd easily be able to fill about 10,000 CF of air. Add $400 to the initial and yearly costs if you also want to do nitrox.
My total time commitment is less than the 2 hours per week I spent going to the dive shop, and I can run the compressor on my schedule. I take care of my dive gear at the same time, so the additional time to deal with the compressor isn't really significant.
Hope that helps,
-B