What is the typical (N.American/ UK) cost difference between greenfield installation of hot water heating and warm air (HVAC) ducting?
Real pricing would depend on the actual type of installation of each system you would choose to install. For example; Hot water systems could be as simple as a series loop system or as costly as a zoned system where every room had it's own thermostat to control the temperature of that area. Simple and cheap would be finned tube baseboard radiation. More costly would be convectors, wall mounted or recessed into the wall, to the very expensive cast iron baseboard or cast iron radiators. Forced air would also have many options, including zoned or a non zoned system, the use of different types of filters, including electronic air filters, humidifiers, ultra violet lights to kill bacteria.,standard or designer registers and grills, insulated or non-insulated ducting, choice metal thickness for the duct-work. Lastly would you also like air-conditioning with that. So, my suggestion would be to consider the actual type of system you would want and have contractors give quotes for your chosen system. Just asking for a price comparison of hot water vs. forced air is similar to asking how much a vacation to somewhere would cost. Answer is " it depends" I hope this gives "food for thought".