Minrothad merchants know that spelljammers are not readily accepted in most countries, and that spacefarers are looked upon with suspicion. This gives them a bit of an edge, since it means space merchants have few other places to sell their goods. The Minrothad merchants can then carry the goods throughout the world, selling them at inflated prices because of there exotic look, taste or usage. They almost never tell their customers it's from space. Spelljamming ships pay the same costs for docking that regular ships do.
Costs are normal.
Several Minrothad merchants have bought minor helms form Kerneth, an arcane who has set up shop in Minrothad city. Knowing how spacefarers are accepted in most countries, they have chosen to use them for expanding their fleet into space rather than planet bound cargo transportation. Some in Minrothad would like to see spelljamming come to be accepted in the Known World. This would allow goods to be moved faster and cheaper, once ships were purchased. Many, though, dislike this idea.