Interesting, around here all tanks are filled using gallon gauge and the bleeder screw. The screw is open and they start filling, but just as the bleeder starts spitting, the pump auto stops. I have had 7.1 gallons put in a 30# tank.
I normally when on the road I fill a tank as soon as it empties. When we get home, I have the last tank used topped off so we always start with two full tanks.
I pay in about $2.70 a gallon, the pre-filled tanks around here are filled to 15# or 75% of normal full, cost $19.99 that equals 3.75 gallons or $5.33 per gallon.