2.The estimated cost for the entire thing depends on how professional your exchange has to look.No one uses random new exchanges these days,only the one's established are trusted.You will have to spend quite sometime in the market,build reputation of the exchange,stand out from existing exchanges and invest a good amount to keep it running for a few months without expecting any profits.
3.The local bitcoin rule : You will end up spending more money for the marketing and SEO instead of development of the exchange.
4.The cost doubles up as you start putting more employees and promoters.Go for the idea only if you're seriously ready to take risks and your project has the potential for the investors to put their money.