Ed in Richfield WI

I recently purchased a house in Richfield, WI. It came with an older 7' X 7' Jacuzzi spa on the rear patio. After replacing both pumps and a number of plumbing components that cracked in a previous winter, the spa is back up and running! The house has an exposed basement (in the rear) and the spa is located about 15' from the basement doors. The Spa is currently configured with a single electric heater. My previous spa (in Florida) also had an electric heater. The cost to run that spa was about $50 per month, not including chemicals.... I'm assuming a majority of the cost was associated with heating the spa. I am concerned the spa at the new house will cost about the same and I would like to come up with an efficient solution for heating the spa water. Since my house in Richfield, WI has gas service I was considering using a standard gas water heater to heat the spa. From what I have read, an outdoor gas spa/pool heater (with a heat exchange) is not as efficient as en electric spa/pool heater. Is this true??? I can pick up a number of used 40gal~50gal gas water heaters on Craigs-list. Most of them run anywhere from $50 ~ $100. Compared to the cost of an outdoor gas heater ($1000 +) an inexpensive "standard" gas water heater seems like a good (and efficient) solution for heating the water in the spa. I'm assuming I can use a small water circulation pump to move the spa water to and from the heater. The gas water heater can be installed in the basement given its close proximity to the spa. My only concern is the chemicals in the spa having a negative effect on the heater tank. If I am able to replace the anode in the tank on a regular basis, doesn't this solution make sense? Please let me know what you think. Positive or negative feedback is appreciated. Ed