My share of gas money on a private two-tank trip is usually $50-$60. I also have to help launch and load the boat, clean the boat, make sure it's iced up, and clean the fish. This adds a minimum of two hours of my time that is spent doing stuff I don't particularly care to do (work). If you calculate two hours of my hourly rate added on to gas and ice money, then it's much cheaper for me to pay a charter operation to do the dirty stuff for me.
On top of that, the charters around here have a divemaster or deckhand (sometimes both) that will anchor or tie in, and set out all the lines. They'll also pull anchor (always fun) and store all those goodies away before returning to the dock. You'll also have one of these guys hovering off to the side, standing by in case you run out of gas, or have a first stage give up the ghost on you.
Most charters in our area provide snacks (fresh-cut pineapple, oranges, chips, etc.) and water at a minimum, and some even cook lunch on the boat and provide soft drinks and gatorade.
It's also not uncommon to have the divemaster clean your fish, and tail your lobster for you on a charter trip.
Even if I tip out the DM at the minimum 10-15%, I'm still nowhere near a "huge" charter cost. I can save money by going on a private trip, but I come out behind once I factor in the labor costs of doing so.