I have no way of knowing if 30% is too much or a fair price for the marketing, security and infrastructure supplied to developers. What is the alternative? If you allow apps to be purchased wherever then you end up with the problems faced with the Android market where malware runs rife.
It just seems to me that it is like the arguments with the Book Store where Apple let the publishers determine the price they wanted to charge for their books. Amazon takes a big cut too and determines the prices to be charged on the publishers' books and yet, it was Apple that was dragged through the courts when their system seemed much more equitable for the publishers than did Amazon’s.