Of course, too many variables is correct, but having read a number of threads over at the boiler forum, I got the sense that people were reluctant to say how much their installation cost, so I thought it might help me in thinking about payback and break-even periods, which tries to factor in all those variables into one neat number.
Just to get an idea of how others approached such a large financial decision. It seems like the answers so far are veering towards a 7+ year payback, but I didn't really know if their pellet needs were in the same ballpark as mine. I always assume payback is faster the more pellets you need, so I threw out there, my guesstimate for pellets is 7 to 8 tons, based upon my current oil usage over the last 4 years, and how my usage changed with my freestanding pellet stove. I'm guessing most people with boilers were using more than 7 to 8 tons, which means their break-even estimates of roughly 7 years, probably means longer for me.
As for all of your excellent questions, one problem is my house is a bit unusual which makes comparisons hard. That's why I was trying to figure out how many tons of pellets people were burning in making their estimates.
Of course I used 120 gallons per ton as my btu equivalent.