Anything associated with the running / berthing of the boat would be the owner's expense whether there were any crew aboard or not. Berthing is charged by the size of the boat, not how many people are on board, fuel costs the same. And wear and tear of the boat? Again, those are the responsibilities of the owner.
I gather from the website that these expeditions are supposed to have some positive social and environmental impact. As a volunteer you are opting to participate in that endeavor and help cover the costs for it as well as doing your share of the work involved. Presumably you hope to get something out of it as well, -otherwise you shouldn't do it.
You can't compare the dollar amount they charge to what someone else may or may not charge for an entirely different proposition.
Fuel, berthing, food, drink are all part of the costs of the expedition that you are opting to participate in. The costs to you are known up front. If you think they are out of line, then don't go.
Expecting a skipper to pay for your way home is crazy even if you can contort the spirit of some laws to back up your argument.
Now, if the sanitary conditions are terrible, the food inadequate, and the vessel unsafe, you have legitimate reason for complaint. Feel free to warn anybody else about that particular operation. I'm not sure why the name of that operation must be censored here since people are free to complain about other organizations if they feel the need.
My sense is that they rely a lot on volunteers and as such the people in charge of maintenance and provisioning may not be all that experienced. Which means that the degree of success in performing their duties can be very uneven. Your trip sounds like a horror story. Maybe all the expeditions are like that. Maybe others work out better. You are trusting an organization that more than likely operates on a shoestring.