imo, I would want a licensed/insured professional to install a GAS water heater in my home, working with gas is dangerous even for professionals. So when comparing prices, keep in mind who is licensed/insured , and who is not a professional. Usually the lower prices "too good to be true", are not licensed/insured, and these are the people you cannot compare with the licensed/insured professional.
As for Handymen companies, if they are not licensed to perform plumbing/gas work, they should not be in your home giving you an estimate.
As for Plumbers using products you cannot find, well that's usually something that can be explained easily. Why should a Plumber warranty a product that was made inferior by a "home store" or DIY store? Thats why plumbers use products from supply houses, they are better quality, and the contractor will warranty his own equipment. Try buying a water heater, and having it installed by a licensed pro. They will not warranty the heater, and if something goes wrong with it, you will be charged to fix/replace it even under a year.
AS for a price Gas water heater have now gone up, figure a 50 gallon i've heard installed in Ny for $980.00-$1180, but this only if for a true professional installation.