Even a billion won't do. You can't write a "platform" from scratch. It just doesn't work. All successful platforms (yes, every last one of them) started out their life as useful tools with limited scope: the original Macintosh, Unix, Java, Rails, even windows. All of these things started out as specific products to fill a specific niche.

The only plausible counter-example I can think of off-hand is .NET. But even there, it started out not as a new from-scratch platform, but basically as a clone of Java with some windows-specific bits thrown in. So maybe a billion is enough if you're just trying to compete, not innovate.