I have not ever seen anything ‘official’ on these. Travel arrangements are made on a case-by-case basis. What one may pay at a fair price may be to another outrageously expensive. A person good at bargaining may be able to convince a caravan master to charge only one ceramic, while another may end up paying two.
And it would also depend on whether or not the person traveling is an asset to the rest of the caravan. A fighter with a good sword might be able to arrange travel arrangments as part of an agreement to help stand night watch, while a commoner with little to no skill at fighting may need to pay extra.
It is of my opinion that travel by means of silt skimmer is always an expensive venture, and dangerous too, but a great way to create adventure!