I am looking into getting a new boiler in our house, which has hot water
heat.

Some years ago, in another house, a relative had a 90% two-stage 80,000BTU
Trane forced-air furnace installed for about $3000. Now my house is larger
and there are some pumps and valves that need replacing and also the
compression tank. But according to the first and only quote I got, just the
difference in price between a 80% boiler and a 90% boiler is $3000 (both
about 135,000 BTU/hr, $5500 for 80% vs $8500 for 90%). In both cases the
same pumps/valve/tank need replacing.

This installer comes highly recommended and I normally don't shop just by
price, but I am wondering why condensing technology should cost so much
more in a boiler compared to a forced-air furnace.