All righty! We're closing the gap.
Please note: the expenses include many one-time purchases - tools, books, irrigation supplies - that won't need to be made again. But I'm trying to be thorough, and I think that over the long term the gap will close and eventually move into the black, even with these big up-front purchases. And more than half of the 16.5 hours was taken up in weeding the yard (still working on it, too), but, since those weeds are going to be reduced to compost used in the garden, I've decided to count it as time spent on the garden.