Hey, this is a great thread! I haven't signed onto
4E yet, but if I do it will definitely be sandbox style.
I think you have to throw out the idea that characters end up with a certain amount of wealth at each level. Like LostSoul said, monsters have wealth according to their challenge, and the characters are entitled to whatever they can get!
PCs in a sandbox game who get to purchase magic need to make different choices than in a standard D&D game. They need to assume that they will end up overmatched at some point, and invest in 'escape magic' or other means of not ending up a snack. I don't know enough about
4E to know how they would go about this.
anyway, I look forward to reading more on this subject!
Ken