I don't think we need to worry about agents going hungry. They can make enough on a few decent deals to keep them in a good lifestyle for a very long time.
I'm sure there are people here who know more about it than I do, but from what I have heard the most common way they earn money is by getting around 10% of the player's earnings and transfer fee.
For example, if a player goes from ICT to Dundee Utd for a £50,000 transfer fee, on a 3 year contract of £60,000 per annum, the agent would get 10% of the total value of the deal (£230,000) giving him £23,000. Don't know how much of it would get paid upfront though, and how much spread over the 3 years.