It's probably tough to generalize, but can you give an estimate of the cost of required materials for the head. Let's say 2 people for 2 months or something similar.
Very, very little! Our costs, living aboard about 50%, have been maybe $3 to $5 per month, for two persons, if I had to dump and reload today. But I don't...
Prior to installing our Nature's Head, I was under the (mis)impression that you were supposed to add more material with each "flush." I wondered what visitors would think of this. But, no worries, the manual says not to. (Pg 13, 2nd from last paragraph.)
When we ordered the NH, we bought 2 bricks of coir at Walmart (the bigger ones have it in the garden center, but not all locations carry it) for somewhere between $6 and $10 per brick. I forget the exact. We hacked one into a pile of chips with a dive knife. Then, following the manual, we added just enough water to soften enough chips to make a couple gallons worth (expanded) and dumped it in. That was most of, but not all of, one brick. Coulda used the whole first brick, but we filled it just to the level the manual said.
We stay on the boat from 1 to 3 weeks, and go home for 1 to 2 weeks. Repeat. After 2.5 months we have yet to add any more coir or empty the #2 bin. The only other cost so far is the paper, which I assume you use anyway. To save bulk we use cheap, single ply "Scott" brand.
We take the urine jug to dump into the porta potty at the end of the marina dock row every 3 to 4 days when we start to see the level visible in the jug (comes above the height of the jug holder of the main unit).
Initially we often used vinegar water as a spray / rinse / flush, but that smelled worse than the poo. Now we only spray to get rid of the (very rare) skid mark.
We love the thing!
Routing the air vent out of the boat without making more holes in the boat, especially where we might get a splash of water was the biggest installation hurdle.
Second challenge was that the old Jabsco was on a platform over the curve in the hull. I had to extend the platform forward to support the NH, and we added a step / footrest in front, because the NH + the platform is pretty tall. (The NH is the height of a standard toilet, unlike the Jabsco which is some kind of kiddie training potty.) Standing on the platform I hit my head on the overhead.
If I had to do it again, I *might* look for a smaller model, just to simplify installation. But the composting head concept, if implemented correctly, is so very much better than the holding tank and twice weekly pumpout.