I think you'll find that air prices vary greatly per LDS.
A lot of this is based on the capital investment and operating cost of their compressor sysem. Many shops have "surplus" compressors that are left over from government military projects, etc. Some have "brand new" state of the art systems that are O2 clean with o2 clean mixing panels, etc. You'll see the prices vary greatly between these setups. Add the extra expsense of 'bank systems' or banking of pre-mix nitrox, and you'll see more expense.
Of course the more fills a shop does, the more wear and tear on the compressor and maintenance, but the more the fills offset the capital investment in the system.
Shops in my area charge $4 to $5 to fill an AL80. Shops in the FL Panhandle seem to charge $5 to $7 for the same tank. I've noticed that shops in some other areas charge $7 to $8 for an air fill. This seems kinda high to me though. But in all reality I think it's more in line with the "real costs" of an air fill for a shop that has a higher end compressor system instead of a military surplus system. Especially once you consider maintenance costs, electricity, filters, labor for work force, insurance, etc.
Of course Nitrox fills only cost more to deliver by the shops.
I do know of a few shops in the FL Panhendle that give free air fills if you buy the tank from them. But you pay "full price" ($165) for the tank (AL80) and get something like 10 free fills or free air for the first year. So to me that doesn't seem like a bad deal.
One way of helping with the cost of air fills is to see if your shop has a 'bulk' price. One of our local shops sells air fills in a 'air card' where you buy 10 fills for $30 bucks. That equates to $3/fill which is pretty darn cheap. Of course that insures you'll visit their shop more often and hopefully buy other stuff. So I'm sure it's a "loss leader'.