Around here it is U-Haul (refill - I do not exchange), 3.99/gallon incl. tax.
My tanks are seldom fully empty, since when I know I'm going to use propane I will usually have the tank filled on my way to/from work when I pass it.
So I'm not sure what a full 20#'er comes to, but I had one empty, one partially empty, and a partly empty 11# tank I use for camping, and all three were filled for ~30 bucks.
I'm not sure if you get 4.5 gallons in an empty 20#, they advertise that they fill them all the way up, but regardless, you only pay for metered volume, and their pump has an inspection stamp same as the gas station.