Pretty hard to put a number on this one. If you manage a couple of nobodies who have been in L.A. for 20 years and are still doing student films and industrials, good luck.
However, if you're Tom Cruise’s manager, you're making more money than 99% of the actors in town. Generally speaking, you drag in 15-20% of whatever your clients make, plus up to 50% of any side projects, if you've played a part in getting them into it. Your client's new "eau de toilette" taking off? They may not be the only one who is suddenly flush (with cash). So the more they work, the more you make. That's what those in the business call "incentive."
If you're employed by a talent management firm, you're getting paid a salary, but it isn't where you're making the bulk of your bucks, and the firm knows it. So they're not going to pay you a ton. Your real reward is the opportunity to have access to a greater number of artists who can make you a rich, rich man (or woman.)