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I sold a tub about twenty five years ago called an "Encon" which was an open thermoplastic tub with no seats, and it had a simple circ pump and heater, no jets, plywood siding and 2x10 wood lip. It was around $500 at the time.It came with no insulated cover, and most of the ones I saw had a blue bubble cover. They would not stay warm in cold climates.I don't know what you could get today for that price. 8-)
We have a SPa-n-a-box and love it. The thing has handled four NY winters...I put it up for about three weeks two seasons ago and only noticed a $20-23 difference in my bill.. guessing it was due to the hot tub.