By the time they kick in there is a contract governing their disposition. No agent would try to determine them beforehand. They are negotiable issues, and they may well be different in the case of each book the agent sells for the author. If you think about it, you can come up with a few other items involving money. Copying costs, for example.
Agents make a number of submission copies of the ms.
Maybe as many as six, possibly even a dozen. That s a couple of hundred bucks. And there s the agent s postage, and her telephone and fax bills. Once the book is being marketed abroad those might be significant. And hey, don t forget her childcare.