Africans

Good lord, you're starting out with some of the toughest things to work on. African stuff has been boiled to death before you even get it. Any grease is going to be cooked into the bone pretty deep. Try whitening them all first and see what you are working with. Buffalo with paste so the horns are not damaged any further than what they might be already. Look up hippo skull in this section as there is at least one that was done. It took many months and came out ok. You are going to have to find something big enough to submerge it in and maybe use 50/50 ammonia with maybe some Dawn added. I don't remember what the fellow ended up using. The other ones with horns you might get them off if you soak them in warm water for a few days and see if they loosen with a few raps of a rubber mallet. I don't think they take the horns off when they are cooking them and they will be dried on pretty good by now.

Charge? Need to cover your time and materials as well as loss of your shop space for other projects for a long while. When you get the skulls, have your friend there when you unpack them (unless he is bringing them to you himself). Examine all of them completely and photograph them and any damage you see. Use good light and focus clearly. Document any damage and grease before you even touch them. Especially look for brittle, soft and flaking bone which means you might not even risk working on that particular specimen other than sealing and stabilizing it after whitening. Boiling destroys the bone structure and even more so when done for too long. Look into a product called paraloid B 72 and look for posts about it on here. If there are skulls that are disintegrating, this will stop it and prevent further damage as well as protect the skull and seal it.

 

Hello from Montana,
If these skulls represent a prized collection, why not just contact a professional skull preparer who has experience. I can't imagine "practicing" on these skulls. Just help your friend find the best preparer in the area who can handle the task. Here in Montana, there are several very reputable, experienced and skilled skull preparers. In any case good luck.
Best Steve