It's not just excellent. It's reliably excellent. You wouldn't use it to describe a fluke. You use it to imply that the person who did the excellent thing possesses the skill or character to be able to do it again and again. When the line is spoken "you're so money" in Swingers, it's an expression of confidence in 'Money's ability to deliver. It comes from "money in the bank." You can rely on him, so your bet is as good as money in the bank.
Think about it, guys.
Urban youth 1: What a fluke! I won the lottery today. I am so money.
Urban youth 2: No, you're not, because you've never won it before and you'll never win it again. If this was your second lotto win, you could then say 'I am so money.'
Urban youth 2: You have educated me. I can always count on you, dude. You're money.