10% difference times the amount and cost of the fuel uses. Do the math.
Lets say the cost of operating the 85% boiler for a year is 800 bucks
So the 95% efficient boiler is going to cost 80 bucks a year less to operate
divide that 4 grand up charge by 80 and you get.......a 50 year payback ratio.
Of course the boiler will either crap out or be yesterdays news or you will be dead by then.
The big problem here is that even though 6000 sq/ft is a big assed house, unless it is 95% glass and uninsulated it ain't going to need anywhere near 360 thousand BTU/HR to heat it. Who did the heat loss?