While the Player Core has straightforward prices for what it costs to hire someone to cast a spell, I've always struggled with what it would cost to hire ritual casters.

After all, rituals have rather big differences from ordinary spells:

• They take a lot longer to cast
• They usually require multiple casters
• Unlike with most spells, success is not automatic (and the more casters you need, the more difficult it will be to succeed)

So how do you price hiring NPC ritual casters? It would make the most sense to scale their daily fee depending on their skill bonus, but what is appropriate here?

And do they get paid for "time and material", or do they only get paid for successfully casting the ritual? The latter is far easier to calculate, while the casters will require a considerable "risk premium" for their service, and they might refuse to even make the attempt if the ritual is particularly difficult.

What are your thoughts on this?