IIRC the rate is about 0.005% of the total item values. There is also an option to have the entire workshop contents of one settlement moved to another at a price. It really draws a focus on the importance of settlements in Survival, as well as the importance of giving provisioners mining helmets so you can spot them easily. It works great and the high costs balance perfectly with the very limited benefits for Survival. You can only travel along the route of that provisioner too, so I can't just go from, say, Jamaica Plain to Sanctuary unless I have set up a provisioner between those two locations. I can't do it from the Pipboy, I have to speak directly with the provisioner. I've set it up so that I can fast travel only if I speak directly with the provisioner and pay a fee.
