I think E-Books are a bad test subject. Theoretically they are a durable good, but realistically the demand for a single public domain eBook is not a large market and easily satisficed.

If you look at real world examples like wheelbarrows (something like 80% of all wheelbarrows sold in the US come from one company) - sure they use a bunch of tricks to charge more for some customers - but the cheapest, basic wheelbarrow is being sold for something close to marginal cost.