In my experience, if there's an option to ask same price everywhere, sellers will do that. So on app store it will cost (for example) 4.99$, and on third party app store without 30% cut, the developers will still set the price of 4.99$.

The only kind of place I've seen prices less than MSRP is discount stores like cdkeys. For example, try to find microsoft windows that costs less than MSRP -- on all stores, first party and third party, the price will be the same.

So, lack of 30% markup is very unlikely to reduce the final asking price by 30%.