Workshops are 10 gold each. (I say each, because workshops are specific to a function. A potion-making workshop can't be used for blacksmithing, for example.) Workshops are also regionally located. If you buy a workshop in New Acarthia, for example, don't expect to be able to use it for production purposes when attending the Tournament in Warchester.
As far as purchasing them, I believe you would simply need to let Logistics know of your intent so they can prepare a workshop tag. Then you'd just need to exchange the gold at check-in for your shiny new workshop tag.