Sometimes you can get better than $20 for exchanges. I have had one place had a limited time where I was only paying $5.
To the OP: Is this a place you are staying at for the season? If so, I'd second the above suggestion at getting a rental tank, rather than keep messing around with 20# bottles. Just the gasoline saved by not having to drive to exchange tanks may make a large tank be worth the initial expense. If a rental tank isn't feasible, perhaps go with 30# or even 40# bottles? I'd say go with 3-5 of those, to minimize the times when you have to go for a fill, plus when down to the last 1-2 full ones, you can swap tanks and then get the 2-3 spares refilled.