Your calculation assumes an overhead of zero: for example you assume that knocking on doors takes up no time.

Plus: is knocking on doors obsolete? I'm not sure you'd be able to get a response in my suburb (not that I've tried - too many gates - too many suspicious people).

And prep, traveltime and cleanup all take time too. for example a mower doesn't magically get to a house and it needs time and expense to run (and where I live you'd often be expected to dispose of clippings - costing time or money or favours).

Trite answers are cute, but they are not helpful.