It's sort of a 'pay me now or pay me later' thing.

In your area of Florida, each time you heat a 400 gallon spa from 80 degrees to 104 degrees (∆24), you'd need around 80,000 BTUs (400 x 8.3 x 24). At 98% efficiency for an electrical heater, or about 24 kilowatt/hours, at about $0.15/kwh effective cost. So, each evening when you came home, you'd be spending $3.60. If you use it 20 times per month, that's $72.00.

Given that the cost of a gas heater is about $700 more than an 11KW heater . . . . well, you'll have to work it out from local gas rates. If you already have natural gas, costs may be only 1/3 of electric heat.