I think it pays for itself in 24 months if you consider the electric bill as if it where a regular space heater cost.
Something to ponder.
30 months = 900 days x 2.30 usd a day = 1860 so if you need to use a space heater 24/7/365
it pays off in 30 months. if you need it ½ the year it takes 60 months.
not counting diff and coin price.