Well on average, an online music store typically needs to sell about 100 million tracks a year to break even on operational costs, and thats for a fairly lean operation.
This frankly sounds ridiculous. So you're claiming a "lean" online store requires $100 million per year in operating costs to run (99 cent songs)? What do they do, hire union workers who individually go to a warehouse, pick up a USB stick with the AAC file, and ferry it to the Web server when you click "buy"? Amazon's operating expenses are about $700 million, and this is for a heterogenious online store with physical warehouses, delivery, huge customer support, auctions, substores, web site hosting, etc
A music store is a glorified download service. The only difference between a pirate site hosting forums, ratings, mp3s, etc is and iTunes is the checkout operation, and customer support.
A "lean" music store should be able to run on a shoestring budget. The bandwidth consumed by 100M song tracks downloaded is trivial, it wouldn't even cost 1 cent to download a file on existing co-los. Once the software for the store was developed, scaling is simpler a matter of scaling the hosting, and scaling customer support.
I guarantee you with 3 months of development time, and <20 PHP/Ruby-on-Rails developers, plus a garden variety hosting service, I could duplicate iTunes's service AND be able to scale to 100m downloads per year, and not spend more than $5 million. And frankly, I'd outsource customer support to India or the Czech Republic, and I bet total expenses per year would not exceed $20million, counting marketing.
The only reason big company stores are so expensive is because corporations are top heavy with loads of employees, managers, and support staff. They invariably end up with too much labor, and the middle management incentive is not to decrease labor. A Sony or Apple store will cost 10x to run than a place like ZipZoomFly or allmp3 because of the corporate inertia of those relatively old companies.
But these costs to run these stores are by no means "fundamental" costs. That's why tones of Web 2.0 startup companies handle millions and millions of user transactions with a handful of employees and hardly any startup funds. It's not rocket science.