We use 30# bottles so always get them refilled. Bought new at Home Depot so I know how they have been used, know the dates on them, and I have each numbered. I keep track of how much goes into each one at each fill so I know how much I used, and can correlate that with local temperatures. Makes it very easy to budget, and I can see if any cylinder is doing something odd. I have 6 of them so I can get through an entire week only going to the filling station once when it gets really cold (-F temps), keeping the interior at 70F.
The going rate around here is about $2.89/gallon. U-Haul tends to be more, but there are places as low as $2.19 if you know where to look. Pilot/Flying-J tends to be convenient but runs around the average. There are lots of gas stations that offer filling services. I prefer places that just measure how many gallons go in and don't weigh them.
Just watch out for inexperienced fillers. I had one stop at just over 5 gallons because "they shouldn't hold that much", He thought they were 20#.
I also had one PFJ where the kid said he could legally only fill two cylinders, not all three I had. He said his manager told him that was the law. I got his managers name, and then I called PFJ corporate. Turned out that wasn't his managers name, and, of course, they corrected the store (while I was on the phone, no less). Never saw that kid there again.
