In my long experience with growing potatoes, to get maximum production, harvest after the plant itself falls over and begins to die. This normally happens here starting in mid-May. Its important here to get them out of the ground before Memorial day because they will rot quickly after that.
I average 10 pounds of harvest per 1 pound of seed cuttings....sometimes more, sometimes less. Price has absolutely nothing to do with why I grow them....nor any other garden veggie.
Simply put, I love the taste of new potatoes. Its a taste that you can not get from store bought versions. What we don't eat out of the ground, we can and/or store in a "dry", dark, breezy location. As long as they are properly stored, i.e. never touching each other, they will often keep until the next planting season...but we always eat them before that. I love the taste of new potatoes.
Spray? I never use it on potatoes. I'm very careful to dispose of the tops, rotate the planting row, and use cover crops in the planned location prior to planting seeds. No need to spray...just enjoy new potatoes.