The return line needs to be laid out properly and installed properly for a gravity recirc system to work. Any jogs that take that line up again will stop the flow. There is very little difference in density of the water at the temperature extremes so it doesn't take much to stop it in the return loop. It doesnt' need much of a pump, but that pump must be either SS or bronze since it is potable water, so that makes it on the expensive side. Operation of say a 1/30Hp pump isn't very much and can be reduced if you put it on a timer so it doesn't run when you have little expectation of needing fast hot water.