$1800 does sound high, but don''t underestimate the cost of doing the job right. For example, while some marine heads are very cheap, the ones that will last a few years cost more. If it doesn''t have a head already, you need to account for the materials and labor to install through-hulls and seacocks. Odor resistant hose is surprisingly expensive. You''ll need a pump and Y valve, and of course all the fittings, hoseclamps, etc. If you didn''t have a head already, you may have some woodwork and/or glasswork. You also need to consider the tank vent and vent lines, the pumpout hose and deck-connector fitting. Even the stainless hardware to bolt the head in place costs more than most would expect. You add it all up and you''ll probably still be short of $1800, but it isn''t likely to be as cheap as you think.
One thing working on my boat''s systems has taught me is that everything costs more and takes longer than I initially expect.
Oh, and I have put a new head in my boat too. Removing the old head and hose was one of the least pleasant jobs I''ve ever done, and if I had it to do all over again would happily pay someone else to do it.