I would like to know how much it costs to develop a website like Yahoo Answers, and what the requirements are to develop such a website.
closed as not a real question by Thomas Owens♦, Falcon, JeffO, DKnight, ChrisF♦ Aug 9 '11 at 12:39
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You need to track users, questions and answers, also certain kind of reputation. But real cost depend on how complex you want your system to be. Building a simple ask and answer site with something like ruby on rails will take you a couple of hours. Scale it to be highly concurrent and so on can take you months.
So, it depends, how many users you expect to interact with (as it happens with almost all websites).
Download Shapado and deploy it. It is open-source, and has a lot of the same features as Stackoverflow.com.