Is it legal to make money of a website developed in Visual Web Developer 2010 Express (which is free)? Or do I have to buy a license or something?
It is legal to make money from a website using that software.
No license is required you are free to earn money from sites developed using the Express tools.
Revenue for Microsoft comes from the (usually SPLA) server licensing costs that hosters have to pay for hosting your sites on IIS.