Well amount of outstanding tokens/coins is a startpoint and than they calculate what they need in $ in terms of investment of the technology/marketing/salaries etc. Total amount of money needed / total coins= ICO price.
I don't like that idea because on the stock market you value the company based on for example net profits of the last couple of years / forecast / intellectual proprerty and than based on that value you have an IPO price. Some ICO's tend to overprice their coin/token and that's why you see for a lot of ICO's that the price will drop after an ICO due to people valuating the company much lower because of failed to meet the requirements (= no product, bad marketing etc.)
i am not talking about the price during the ico period. i am talking about the opening price in a platform after the ico finishes.