I play minor league baseball (Astros). I have heard so many horror stories of agents taking huge chunks out of contracts. For US based players, a typical agent takes 3-5%. Another big issue is the Big League Advance- basically mini capital firms that pay players that are 18 or so a chunk of money (low as 20k) for 20+% of their future earnings. Preys on players who maybe didn't sign their initial contract for very much, which is very common. Fernando Tatis Jr. was one of the biggest examples of it coming to fruition.