It varies from breed to breed and shoule agreed in writing between the stud owner and bitch owner before the mating, and what the terms of payment free return etc are. In my breed stud fees are pretty standard, and the price of a puppy.
We use to charge either the price of a pup or have first litter pick.
It depends on who you are doing business with reference studding.
Bewarned its not always straight forward and if they lock up don't spray them with water as the old saying goes your better to let the dogs part normal.
Great damage can be done if silly practices are used.
They will part when one or both relax so just watch calmly and don't interfere.
Thats my advice but its upto yourselves I can only try and point you in the right direction.
It depends on the breed. It's inadvisable for both dog and bitch to be unproven. Certainly no more than half the price of a puppy, with a free repeat mating (if both dog and bitch are in the same ownership) if the bitch misses.