Fin And Mrkts

How much does it typically cost a company to "go public?" In other words, if an established private firm wants to sell shares to the public for the first time, how much will the investment banks charge to assist them in this? Once a company stock is public, what responsibilties/expenses does the company now have that it did not have before? Why go public in the first place? (300-500 words--cite all sources)