My KA-6 was $4500 in the summer- 3-4 years back.
Big John. Is your current set updoing the hot water also? You may want to go with the KA-6. It's been bullet proof for me so far. I'm very happy they did not have the KA- 4 when I was looking. It probably would not have been enough for me this year being as cold as it has been.
Boiler in basement of 4 year old modular (3500 square feet plus 1700 square foot unfinished basement that gets up to 76degrees from boiler radiation
)with good insulation and windows and doing the hot water. 7.5 tons a year. As they say.... your mileage may very!