I agree that propane exchange is not a good value simply based on the price. Didn't realize you weren't getting a full tank, so that makes it even less of a good deal. But... this thread was about 20# vs. 30# tanks. I do normally take my tanks to a local feed store that fills by the gallon for a very reasonable price. I do this for two reasons - I usually bring them in with some propane still left in the tank and it's cheaper.
However, if I failed to plan ahead and need propane while I'm on a trip, it's awfully convenient to be able to just pop into a local gas station/convenience store and exchange a tank rather than driving who knows how far for a fill station.