>Not to mention that gas heaters are simply more efficient for heating to begin with.
Are you sure about that? A heat pump with a COP of, say, 2 is 300% efficient (it is said to have an Energy Factor, EF, of 3). That is, for every Joule of electrical energy supplied to the pump, 3 Joules are added to the water tank. On the other hand, the combustion products of a natural gas heater need to be exhausted outside the building. Unless they are first cooled to the temperature outside, the heater is less than 100% efficient.
Where I live, a Joule from the electric company costs about 2.5 to 3 times as much as a Joule from the gas company. A quick Google search indicates that this type of water heater typically has an EF of 2 to 2.5. The cheapest gas heater I could find on the Home Depot website was over $300 (twice your Mexican one) with an EF of 0.62. So it wouldn't surprise me if the energy cost of an electric is on par with the energy cost of a gas heater, and it would surprise me if the electric actually used more energy. I don't see how you would save thousands of dollars in energy with a gas heater: Even in the worst case (a 2 EF electric heat pump vs. a 0.62 EF gas in a location with a 3x energy cost ratio), the gas only just breaks even.