> probably be created for less than a billion dollars
I don't think I've worked anywhere that's put more than $500M into a single software project. That is "huge" cost.
(Except maybe Yahoo! but even then I'm not sure.)
It's not what they put on the project in one go. It is the accumulated cost of developers, infrastructure and most importantly -- the experience and expertise that has been cultivated over decades. Do you really believe you can get a bunch of developers with limited experience to build something that is comparable to iOS/Android in a couple of years?
I don't, no, but then I wasn't claiming it was possible, especially not for "under a billion dollars".