Russ you are right $18 at menards and if you read on the side of the tank,you get 17 pounds.
As for me, TSC and Menards are not out of my way. It took me as long to exchange at menards as it did to refill at TSC.
Another nice thing at TSC, they will top off a tank and you get charged for what they pump in your tank.
I think it's a good bargin. Hope it's helpful to some.