For everything in the game, it's a combination of MA and the players that determine the MU. Yes, in the end players decide how much something is worth, but MA certainly has a hand in this by 1) controlling the drop rate, 2) controlling the usefulness of items.
Introducing a new and desirable crafted item will create more demand for the resources that are used in that item - even if they don't touch the drop rate, the MU will go up (ArMatrix did this with some resources). Conversely, they can also make the MU on a common resource go up by simply lowering the drop rate, without introducing any additional new demand for the resource.