I know there has been the disucssion of the refilling of 5# tanks in the past. I think the high price is due to the pain in the A$$ of refilling when you walk in instead of a simple tank exchange. Or it may be that the small quantity for some reason is just priced higher for motivation to aquire a larger tank. My Father-in-law just had his 'frige go out and he needed some dry ice. Air gas had 10# for like $22, or 25# for $17. Go figure. But through talking witht the guy a "special" pricing was found for 25# for like $8. I know I'll be using my father-in -law's new found connection for my summer cooling needs.
But on the topic of tank supplies:
I have no idea about the refilling of any co2 tanks, other than the 1 time I passed off an extremely old outdated 20lb steel tank at Air Gas and for $20 got a filled new aluminum one. I know there is killer pressure in these tanks and wouldn't attempt this without some expert advice, but couldn't a smaller 5# tank be refilled from a 20# tank if the right fittings were had? I know both will be at lesser pressures than each being fully professionally filled due to that silly law of equilibrium, but in a pinch couldn't they both be hooked up and valves opened for a quick refill?
If it is possible, I wouldn't mind helping out and "passing gas"

to another brewer. I drive past Air Gas at a minimum 2x per day. I could get the 20# refilled more often, and possibly get in better with the guys there. Maybe even figure out some way to get our tanks refilled, or topped off at meetings for only a couple of bucks.