Interesting...we have a gas heater for ours and it cost $3K to replace it when it went kaput.It will be tapped into our existing in-ground 500 gallon propane tank. I don't expect we will heat it often, as it will probably use a lot of propane. Just for special occasions.
The pool heater & installation part of this was $3300.
I assume that since you said you are not going to use the heater often that you don't have plans for a jacuzzi? Best part about our pool is winter nights when it is like 50 degrees and we fire up the jacuzzi. Fun stuff!