Update - finally made it over to Robert's. $12 per 5# swaps, and $18 per 20# swaps. They don't have 10# cylinders. About $140 for a 20# aluminum cylinder, and a dollar or two less for steel. I'll probably end up buying a 20# via Evilbay (about $110 shipped via beverageelements, wonder if it's slightly different shipping through their website...) and refilling once every 18 months or so. It's a bit of a drive out there - just west of the Gboro airport west of the city - and it'd be nice to minimize trips.
I did ask the guy at Robert's if they had any low-flow metering valves. They didn't (but could order), but the guy actually asked me if it was for a fish tank. Impressed the heck out of me.