You can buy him from the Marketplace for 2 hours at a time. You can explore as many times you want if you start the exploration within the 2 hour time frame. Example: you could send him to the Butterfly Oracle (1 hour to explore) twice on one hire. But if you exceed the 2 hour time limit you must rehire him from the Marketplace. The very first time that you hire him he will be free, any other time and he costs 12 whole crowns. 
The probability of getting a crown when Fredric explores is shown on the right side of the screen just like any other item.
When you first employ his help, Yvette will help you along the way with quests to show you what Fredric can do. I wonder if Yvette is going to cheat on Rafael with this weird ghost looking guy LOL.
Fredric's lockbox is at the Well of Wishes.
Fredric's key can be found by sending Yvette to the Dragon Ruin.
