A big chunk of the money removed from Cornwall by the Duchy does not get to recirculate in the local economy.
I've forgotten the actual figures off the top of my head, some of the sums involved in moorings were astronomical and parasitic.
I'm all for the Royal Family as they don't actually cost that much, the Duchy are costing some people a big amount of money indeed. I'd rather they got pocket money from the state and kept it at that, rather than attempting to be business people and missing out on the service bit.