There are many factors in what it costs to heat the water in the pool or spa.

What you are probably asking is, "How much will my heater cost to run?"

A 400k BTU heater will use that much in gas in one hour. Most utility companies charge by the therm for natural gas. There are approximately 100K BTU in a therm. You can do the math from there. Find out what your company charges per therm and multiply it by 4 and you will get the approximate hourly cost. You can contact your utility and they an give more precise figures, but since you don't ever know exactly how much water you have in a pool, and the amount of energy in a therm can vary depending on supply, these figures are close enough.

If you are using propane, there are about 100K BTU in a gallon, so a 400K heater will use about 4 gallons per hour.

A 15000 gallon pool will have about 120,000 pounds of water in it. It takes 1 BTU to heat 1 pound of water 1 degree. A 400K heater, at modern efficiencies will put about 320K BTU into the water, more or less. It should be able to heat 15000 gallons about 3 degrees per hour. This does not take into account any heat loss from the surface of the pool.

In warmer weather, most spas will heat to 100 degrees plus in about 10 to 15 minutes or less with a 400k heater. In colder weather (do you have that in AZ?) maybe 20 minutes.