Cheapest place -by far- in our area is Sunbelt Rentals. Looks like they're all over the Dallas area. They'll fill the 20 lb grill canister for $12 tax included, and give you a 5th fill free card, which brings the overall cost of 5 fills down below $10.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.