Interesting. We keep ours at about 95° and have the filter cycle timed to go on at night only when the rates are lowest. I turn it up to 100-103° about an hour before we're going to use it - which is maybe once a week. Installed the tub last year and I didn't really notice any big change in electricity bill, I am sure it's costing me something, but I would have noticed $40 a month.
I am sure the type and size of the tub have something to do with it as well.
Electricity aside the thing is a PITA. Testing the water (couple times a week), adding chemicals, cleaning the tub, cleaning the filter, etc...all things I'd rather not spend my time doing. This reminds me actually, I am suppose to go buy a new filter on Saturday