How well do whole house humidifiers work?
Honeywell humidifiers are really good - with experience with one in last house.  Our heating costs went down by 15% and our house didn't have static everywhere in the winter.  Mine was a steam too and it cost me about $400 total after a couple of quick trips to Lowes.  Took me about 3 hours (counting the 2 trips) to install.

So let's say it takes a whole day for the pro to install (8 hours) - I am not an hvac pro.  Is it worth $300 an hour to get this done in your area?  Also maybe your unit costs more but the unit I got had great reviews and near top of line... 

Also I think this should be installed on furnace not garage.  I don't know your whole setup so I don't want to call out the company.  It doesn't make sense to me that a line from humidifier to furnace would be run because that has to diminish the steam going in and may make the humidifier work constantly... But I do understand that you don't want plumbing to freeze - which I am guess it doesn't in your climate being that the water heater is in the garage....

So I would run a water line to furnace and install it there.