@TheBodo The base fee represents things like upkeep on the crafting station, individualized supplies that the game hasn't bothered to detail, like resins, dyes, high and low grit sandpaper, oil/grease, dusting, cleanup, etc... All the costs that would be involved with continuous use of a crafting station beyond just building it.
This is an important Gold Sink for the game, allowing Gold to establish a valuation and not become so overabundant that it has no real meaning. Previously, gold was so undervalued because of its commonality that all prices in the market were way inflated, and only going up, as gold became more and more overstockpiled. Finally, gold is being sinked by more than straight town/house upkeep and harbors, making the amount of gold available overall in world more managable, and the economy of the markets will stabilize because of this, and other steps that will be taken...the balance just has to be found.