Just FYI, my marginal electricity rate in northern California is 40 cents per kilowatt-hour where I am in the 30-40 kWh/day range in usage. This doesn't change the conclusion of your analysis however. It should not cost anywhere near $1000 to heat the spa. It also probably does not need water replacement any more frequently than every 2 years, maybe a little longer if properly managed (unless the bather-load is higher than one person for one hour or so each day).
Also, it's BTU, not BTUH. BTU is an amount of energy so in the same units as kWh. Kilowatts is power (rate of energy per time) while kilowatt-hours is energy.
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