Durasell is right. Let me tell you. Real agents don't mail in samples of your work. They pick up phones to editors' desks.
Usually, 15% domestic, 20% international is standard. Some will ask for 20% domestic, 25% international. That's not unusual either. If it's more than that, it might be a little fishy.
If they want to charge for photocopying, telephone calls, etc..., they're probably fake. Most real agents absorb the cost because it's part of doing business. Just like we absorb copying and printing, postage, etc...
Actually some reputable agents have started charging clients for photocopying and postage, something that would have been taboo a decade ago.
And, I'd add this: the current book market is the worst it's ever been in modern publishing, i.e. post-paperback revolution.