I'm struggling to see how Twitter is of any use in finding customers.
On the other hand spending on Adwords links me directly to people who are looking for my products. For me it is consistent, reliable, directly measurable and profitable.
If you have anywhere near a reasonable volume your payment processing shouldn't cost more than a few percent, nowhere near 30%.
Hosting costs aren't going to go away, you will still need a web site, you might be able to save a little download bandwidth but that hardly breaks the bank nowadays.
I don't think Apple are being greedy, I'm sure that is a reasonable reflection of their costs, I just wonder how it will add up for developers like Panic or Omnigroup. If their volumes double then it makes sense, if they only increase slightly or not at all then it doesn't.
If Panic and Omni find that all they have done is reduce their revenue by 30% then they won't stay long. I'm not at all sure the extra volume will be enough to offset the cost, especially if the store becomes full. It is going to be interesting.