I have to disagree. While there are many instances in Dune where the Guild is cursed for its extortionate practices, it almost always relates to the price of transporting (military) personnel. As a contrast, this quote from Duke Leto:Anathema wrote: - space travel is expensive, and makes it unpractical for the rank-and-file members of the Landsraad to travel for every other issue.
I see two possibilities. Either items like lumber, dung and rice are exported to barren planets that cannot possibly produce these items themselves, to be sold at extortionate prices.
"Few products escape the CHOAM touch," the Duke said. "Logs, donkeys, horses, cows, lumber, dung, sharks, whale fur -- the most prosaic and the most exotic . . . even our poor pundi rice from Caladan. Anything the Guild will transport, the art forms of Ecaz, the machines of Richesse and Ix.
-- from Dune
The second one, which I like better, is that Heighliner transport is very cheap, comparable to cargo ship transport of the 21st century, and it supports a densely interlinked network of galactic commerce. It's simply cheaper to import Caladanian rice than it is to grow your own, just as in Western nations, it's cheaper to import rice from East-Asia than it is to grow our own.
From this, it would follow that the Guild intentionally charges extortionate rates for troop transport. This makes it difficult for a conflict to spill over from one planet to another, which encourages stability, which in turn is a great benefit for everyone—except for that nagging human genetic drive to spread out across the galaxy and plunder and burn and screw.
Of course the Great Houses curse this policy, because it gets in the way of their petty conquests, and there's always the possibility that the Guild may choose to favour one of them over the other.
Does anyone else have thoughts or quotes to support one view over the other, or perhaps a third possibility?