I think the focus on "30%" is a little mis-statement. You can pretty easily argue that 30% is too high, but clearly they aren't going to pass along the 12-15% or whatever it actually costs them (assuming 5-7% for payment, a few more % for server costs and bandwidth, a few more % for customer support and developer support) -- clearly they're going to value their monopoly audience attention at some non-zero number. So what does that bring us to? 20%? 25%?

The real issue isn't the few percentage points the devs would like to claw back, it's with applications that have recurring billing and would prefer not to give a percentage at all. Netflix doesn't want to cut Apple in on subscribers. Neither does Amazon with Kindle. Any percentage based system is going to be unfair to someone: if they take a percentage on upfront purchase cost, Apple/Google will get nothing. If they take a percentage on subsequent revenue or subscription revenue, it'll seem disproportionate on an ongoing basis to the value the companies supply.

Making matters worse, it is not especially feasible to come up with a different rate on a case-per-case basis without grossly distorting the market and magnifying existing first-to-market advantages in an undesireable way.