Re: Installing a traditional head from scratch
I just replaced my whole head system except for the through-hull. I replaced the head, all lines, the holding tank, and re-routed the holding tank vent.
It was a solid day of work and cost a little bit less than $1000. The two big expenses were the head (about $250 for a Raritan PHC) and holding tank (about $350 for a 18 gallon Sealand with custom fittings and two dip tubes). The rest went to high quality waste hoses (>$10/foot) and fittings. On my boat the run from the head to the holding tank is quite direct (the holding tank is in a lazarette just aft of the head) so installation was simple. The overall size of the lazarette made it easy for me to choose the shape and dimensions of the tank without having to get anything too complex. Those two factors could greatly influence the cost of installation.
I suggest reading the articles from Peggy Hall on ventilating holding tanks before designing the layout inside your boat.
If the boat wasn't designed for a holding tank at a minimum you'll need to T an existing through hull for water or add one to the boat. You'll also need a dump through-hull if you want the ability to dump when offshore.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior