I sell software for servers directly on my site, as well. I do pay merchant service fees (which I'm also somewhat grumpy about how high they are), but the total amount of "fees" I pay is remarkably lower than 30%. If I factor in colocation costs, server costs, etc. I end up with ~11% total. (But, we're also supporting Open Source projects with a million+ users, so we have a lot higher infrastructure costs than we would if we only had to serve our few thousand paying customers.)

Even if I add advertising costs (because some people allege the app stores provide a customer base), it's still less than 20%. But, I could distribute dozens of times the amount of software I'm selling with the same hardware and colo (if the market wanted to buy dozens of times the amount of our software currently sold).

So, it's true that there are costs no matter how you sell your software to customers. And, I might be willing to pay more for things that would make my customers lives easier. But, 30% is...it's just outrageous, to me. Apple and Google are among the richest companies in the world. Their margins on running these stores is obscene.