There are an infinite number of ideas out there. I think that’s part of what keeps people excited about technology and mobile apps in particular. Given a set of capabilities and restrictions people can come up with really creative and interesting concepts. Some of those concepts can become valuable if they’re developed and made real (some might say manifested). But what is the value of that concept? What is a fair price for an idea?
Surely there must be some precedent for this, but damned if I can find it. Music and literature royalties are probably similar but the amount of work required to create music or a book is considerably more than it takes to say “I could use an app that does X”. I think that a percentage seems logical but the actual number seems like it could vary a lot (see the Wikipedia article for royalties for example). You could also use a base amount with a percentage or a scaled percentage or just a flat amount. I’ll be looking into this further but if anyone out there has any input throw a tag on it for “app idea compensation” so google can find it.