It's a gamble either way.
Going now he likely loses millions, as his draft position will without a doubt fall drastically.
If he comes back to South Carolina and doesn't have a good year his draft position will fall drastically.
Guess he figures he had better take what money he can get now, and gamble that he will perform well enough to garner a big contract down the road.
I really feel for him. He's a great player, and by all accounts a very good person, who just never has been able to get a positive break since he arrived in Columbia.
Perhaps a victim of the Chicken Curse? (this line posted very much tongue in cheek)
http://bleacherreport.com/articles/1425507-marcus-lattimore-insiders-reportedly-believe-injured-rb-will-enter-nfl-draft