Well it depends, most of this week’s is archive material but there are brand new ones like one of the confusingly branded X-Men Gold books. It’s almost certain they’ve made the production costs back on the floppies on those though so they can’t really lose money.
From my knowledge there are no overheads at all with hosting a digital comic. Comixology host and prepare the guided view and then keep 30% of the price (for small press stuff via Submit they keep 50%).
The big question that Jim raises and I don’t know about is royalties for creators, they will literally make a penny as far as I can see.