Although it has a few "battle scars" from age and use, the heater that came with our pool is over 20 years old and still works great. I wouldn't be the least bit surprised if your heater did as well. However, after not being used for six years, it could benefit from a service call. The most likely things I would expect to see are dirt and/or spider webs throughout the unit. Gas heaters (applies to Natural Gas [NG] and Propane) require a relatively precise ratio of air to fuel in order to burn cleanly and efficiently. Any dirt and webs could impede the required airflow to the unit and disrupt this ratio. If the dirt/webs are present, the unit will likely fire-up but it will not burn at peak efficiency. In extreme cases, it's possible that the unit may not fire-up at all. The second most likely thing would be a fouled/rusted igniter from years of non-use. Both of these issues are easy to fix. My heater simply required a good cleaning and, being quite comfortable working on this type of equipment, I did that myself. If you are not comfortable doing that, I would definitely hire a pro.
As for the cost to run I can give you an approximation. I have a NG heater. Using my NG consumption costs, it would cost me about $0.87 per hour to run a 150K BTU heater. Propane would cost about 2-3 times that amount or anywhere from $1.74 to $2.62 per hour to run your heater.
I wouldn't give up on the heater, especially if it appears to be in relatively good condition. If you decide to get it serviced and run it occasionally, I would get a cover or at least a tarp to put over the top of the unit when you're not using it and after it has cooled down to keep water and much of the debris out of it. To guard against spider webs, spray an insecticide around the base of the unit periodically. Of course, using the heater periodically will also deter the spiders from setting up shop in there.