It seems to me that there's been one thing missing in this whole discussion. Yes, he did something really wrong, and yes, he has to make up for it -- but he needs to know that you still love him, NO MATTER WHAT, and that you will forgive him, even if you will not forget.
Kids that age internalize a lot. Leaving him home from a family trip says "we no longer love you enough to include you." I know that's not what you mean to say, but that's what he'll hear. And feel. And it lasts a long, long time.
I agree with others, he has to pay it back. Not just with money, because obviously that doesn't mean a lot to him, but with his stuff. If you're not comfortable selling it to recoup the money, take it away, Leave it all at grandma's, and make him earn enough to not only pay back the money he took, but an additional amount to pay to get back the equipment.
Make a contract with him, including what caused the problem, the actions he must take, and by when he must take them, and punishments for missing those deadlines. Insist on his participation in setting up the contract. Include methods of checking and oversight. Throw in everything you can think of -- he's probably thinking, even if he never says it, that he deserves whatever he gets, and he won't say anything until he starts to feel that it's really unfair -- at that point, you'll know you've gotten through to him. When you asked him what punishment he thought was appropriate and he said, "I don't know, you're the parent," he's telling you that he doesn't really understand the seriousness of this. Or that he does, but hopes you'll be more lenient than he would be toward himself. In either case, he need to have it spelled out, in black and white, posted on the refrigerator, what he did, what the consequences are, how he'll work himself back into your good graces, and what happens if he doesn't.
No matter what you do, though, you have to reassure him that you still love him -- hate the action, love the kid. He can have consequences, reduced activities, no souveniers, whatever, at Disney, but he's still part of the family. There are some things that he'll never forgive you for -- embarrassing him, treating him like an outcast -- and those things will teach him lessons you don't want him learning. And there are some things that will teach him that you will always love him, but that it's very important to you that he grows up to be a moral, caring, honest person.