Ceremonial Eldritch Magic in the Economy

I'm planning on running a campaign where the only magic accessible to humans is Eldritch Ceremony, but one question has me stumped.

How much would an NPC charge, for casting an Eldritch spell? Given the time, risk of failure or mishap, and potential for Corruption, it seems like the availiability of spells should be sharply limited relative to a world where Clerics and Mages have reliable casting multiple times per day. On the other hand, use of cadres of assistants and careful casting can mitigate the danger, but that's a lot more people you need to pay. I didn't see any direct discussion of this in the Heroic Fantasy Handbook, but I'd love to know more about how you guys have run it, Autarchs and otherwise.

I've adopted the Arcane costs and availability in my campaign and haven't thought about it too much.  That said, the most commonly purchased spells for my group have been things like Remove Curse, Cure Disease, etc - most of which are White ceremonies of 3rd level or lower. The consequences of botching a methodical ceremony of 1st-3rd level are pretty tame (nonexistant if using an applicable talisman).