1. Setting up a site such as Answerbank is quite complicated and unless you are very conversant with program writing, you'd have to employ someone to do that. (not cheap).
2. Answerbank makes the moolah with advertising. Often advertisers have access to each page's number of 'hits' and their advert rates are set by this.
3. There is really no legal cost involved as such, but some type of insurance should be considered in case of public liability.
If you want to set up a simple hobby site you can make quite an impressive page or two with Microsoft Word, and save it as HTML. You can then post it on a free host or most ISPs will host it for free, providing you have an account with them.