Too many ''it depends'' modifiers. In 3E the cost of hiring a sage was 2 gp. I think most of Oghma's priests would give mundane information for free, special information 10 gp/priest's HD or skill ranks. They'd not give top secret information such as the location of the Harper safe houses.
It'd be similar to how much does it cost to get information in our world.... I can get where the local movies is for free. I can get how a refrigerator works by buying a book at my local bookstore. If I want to learn about enriching radioactive materials to weapons grade without being a member of the military, I do so at possibly risk of life and limb.
The Dungeon Master's Guide has guideline costs for NPC services.
Dungeon Master's Guide II expanded on NPC service costs. It was dependent on the skill modifier of the NPC, and at some point they begin charging exorbitant fees for their expertise. By 6th-level, they charge a flat 500gp fee on top of the original cost. By 11th it's 1,000 fee and 2,000gp from 16th and up.