My recommendation is to establish the %age markup based on file sizes, since that would be the basis for infrastructure costs such as storage and bandwidth.

30% is definitely high. I understand 30% has become a buzzword these days, but in reality it cuts a lot into merchants' margins. You may want to research on current digital goods market sites such as e-junkie, payloadz etc. Most seem to have a combination of monthly pricing and a "low" per transaction fee. Monthly fee certainly going to be a barrier for casual merchants.