All in, with all distribution costs, I pay $1.24/CCF of NG. 1 CCF is approximately 1 therm. I did the math a while back and to run my heater for an hour costs me somewhere around $3.50. For us, a big NG heater was a no brainer for two reasons: 1. The pad already had the appropriate gas line size running to it, and 2. we wanted to be able to decide "hey, let's use the spa" and get in 20 minutes later. A heat pump would never afford that luxury, so it was out. I went with the Raypak/Rheem because a number of folks on here in the industry reported low warranty claims and general ease of parts availability (and since I was going to install it myself, I was going to lose the warranty anyway, so reliability was important). NOT having some loud blower was another win in my book.
We've had it for about a month, and unless I lived somewhere with no gas service, I can't see myself ever getting a heat pump living in this area. The NG heater is just too convenient, not to mention our electric rates are about to get massively jacked up.