More to the point, an electronic copy is a sale to a different demographic...
The price of the printed books precludes individuals establishing significant libraries - no such restriction necessarily hinders the digital realm... and, hence, the potential market size should be considered when the price is set.
I think you have a bigger point there outside of the textbooks scenario.
When I grew up my family had a decent-size personal library, so I got into the habit of picking up books for reading with some frequency. Once I was living by myself I quickly realized that, after a few dozen, books have a larger ongoing cost besides their price of acquisition: in terms of space and preventing deterioration they're quite expensive.
They're heavy, require special furniture for storage, take up a lot of room and are vulnerable to heat, humidity, bugs, animals and all sorts of other environmental factors. And they're few specific editions are commodatized enough to be truly disposable (well, unless they're really bad), or rare enough to amortize the overhead.
Textbooks and reference material at least have the virtue of being a quantifiable investment - i.e.: you usually only *need* a few of those around if you have a library around, and they're expensive (and resaleable) enough, and reused enough, to be worth the space and care. So I don't see those changing that much. But for most people most of their *reading* material, both by quantity of books and frequency of use, does not meet that description.
Over time, I suspect physical books will inevitably be moving back to their original social place as cultural collectibles or luxury items. That doesn't mean people wouldn't buy them for aesthetics or personal reasons, or that the prices will be unreasonable - but like music records, mechanical watches and fountain pens, they're vulnerable to be displaced by the raw efficiency of the alternatives.
The 'problem' will not be that people buying paperbacks at 10-20 bucks today will demand a better e-book value. It's that most of the people willing to buy and read each e-book - at whatever price the market bears - wouldn't have considered buying each of those physical books in the first place. Once your demographic changes that much, your old price differentation stops working and you need to price for the new market (or your competitors will).