Crystal, I agree that you should lay out your expectations so everyone is on the same page. It will be beneficial to not only you, but your BMs as well. However, asking someone to sign a contract is more than a little offensive. Since they are your friends, I would think that you would trust them.
As for what you friend did to you- well I think she wasn't a very thoughtful friend in asking her BMs to spend that much money when she was having such a low budget wedding that didn't even materialize. Obviously this experience made you think about what kind of bride you want to be and what you expect of your friends. But taking it so far as to make it a business transactions takes that "specialness" of having the people who mean the world to you share in your day.
If money is such a huge concern and you are worried that your BMs will not fulfill their obligations, why not just scrap the idea of a bridal party? You can still involve your friends in your wedding without the headaches bridal parties can bring.
Or if you are determined to have a bridal party, why not get together with your girls, have some cocktails and apps and lay down what you expect from them. It's as easy as that. Signing the dotted line is a whole other story in my book.