The point was (is) that generally, when a plumber supplies the materials and there's a problem during the warranty period (of his labor, often one year), it is fixed with no further cost to the original purchaser, regardless of what it cost. Paying for it twice, at whatever rate, is overcharged. the fact that it started leaking the same day reinforces the point that he should have resolved it at no cost to you.
I fully understand that many contractors price in the possibility of a warranty replacement into their total contract price.
Still doesn't answer the question I asked, whats the typical contract price to supply and install a 50 Gal AO Smith heater?