First of all, you gave the estimated heat load by saying 2-3 cord. That includes a hefty amount of ground loss (yes, even good line loses a lot of heat), so likely closer to 2 cord actually going into the building. As I said above, I have never paid more than $1.09 for LP. Ever. If you're paying market price of $1.55 that's entirely your fault for not pre-buying when the price is low. For grins though, let's try again with the 2 cord number AKA 25,000,000 BTU and 80% efficiency from the Modine which makes 73,200 BTU per gallon. That comes out to 342 gallons of LP. So Even if you were for some reason paying $1.55, that's $529 for the winter. Seeing as you already have the Modine in place and I can only assume it's still connected to the tank and needs nothing besides more propane to run, at $529 a year it's still going to take 12-15 years to pay off a new OWB IF the wood was completely free, your labor is worth nothing, and pump took zero electricity to run- which we all know is not the case. Hell, just the pump alone takes $30-120 a year to run depending on motor.
All I'm saying is weigh the options. The majority of us burn wood to save money; maybe have a little fun and get some exercise. Don't let the "freeness" of operation let you lose sight of the huge initial investment and overall cost of each over the lifespan of the stove