what makes you think he is making a premium on boiler ? a lot of people decide how much they want to make on job for how long it will take then add on the cost of materials . So it could end up costing you more as he will prob get boiler cheaper than you and just knock off cost of boiler from his quote .
Do you also want to supply all the individual fittings , screws , rawplugs , flux , solder, cleaners, inhibitors , fillers, cement , wirewool etc etc