Honest answer, it's the practical knowledge (from experience) that if there were competing app stores, it would be nowhere near that high, because there's no way the costs for Apple are anywhere close to that ($30 for a $100 app).

Now that being said, a market-based solution would probably be structured very differently, something like $1 a year per thousand installs, plus $200 per any app update that changes features (isn't just bugfixes/performance), plus a 6% cut of sales. It's highly unlikely that paid apps would be able to subsidize free apps the way they do now.

So while there would be winners (like Basecamp), there would also be big losers (hobbyist/independent free apps).