Also, as has been stated, the cost of the heating appliance has climbed over the years - you can easily spend one or even two thousand bucks to get a mid-sized heater. And then the gas line will have to run from the meter to the heater. That gas line will need permits, and a pressure test, a trench, and finally plumbing from the spa to said appliance.
And as I mentioned above, a pump must be pushing water through the thing for it to work.