Maybe it's because of capacity. From checking today's fish count, most Bay Area boats went out with less than 20 anglers, while most SoCal boats (excluding the 6-pack charters) went out with 20-30 anglers, and even the boats with less than 20 anglers still have the capacity to carry a far greater amount. https://www.sportfishingreport.com/dock_totals/boats.php
Cost is generally a reflection of the local conditions. Higher expenses, fewer customers leads to a higher break-even point cost per customer.
I don't miss the upward spiraling cost of, well, everything from living up there.
$300 for a trip sounds very excessive. I follow a lot of the Bay Area charter to see what's biting but I'd rather get some live bait and relax at a pier, thats just me.