Really don't know what to tell you except a few general things. The $1800 sounds real steep for laying a pipe. Code probably says gas pipe has to be at least 18" down. You could rent a ditch witch and do the trenching yourself. Once the ditch is dug then get a bunch of sand and lay a couple inches in the trench, lay the pipe, then a couple more inches of sand then refill. Sand protects the pipe from rocks and so forth. For a run that long the pipe diameter is probably over an inch because of the distance you have to flow low pressure gas. There are tables for this info. This is what I did when I had a propane heater but then again, no codes at my place.
As for the tradeoff of propane vs. heat pump, I have no idea. All depends on propane cost and electric rates. You're pretty far north so a heat pump for heating a house in the Winter probably won't make sense but for a pool you're adding heat when the outside temperature is high so the heat pump efficiency is high. I have no experience with heat pumps for pools.
Hope this helps a little.
Al