Well for me, the e-reader paid for itself with just all the gutenberg downloads. I am one of the people who actually likes a lot of the books I have from PG/MR and all the public domain sites. I was actually buying the physical versions before e-books came around.
I suppose it depends on a person's preferences, but if the expensive and hot bestsellers were on my MUST list, I'd probably not have bothered with an e-reader.
Nobody specifically seemed to say, "oh it pays for itself", but it seems to have always been the experience that when you pay a premium for the startup costs of a fancy gizmo, eventually the reason you bought it in the first place comes to fruition. Things like bread mixing machines, stereos, CD players, etc etc, you pay the premium for a reason, to balance the cost in other ways.
The fact that I rarely buy physical books does not mean comparing the cost for me, is meaningless. They are essentially supposed to be the same thing, same story etc... so yes, I compare apples to apples. If my goal is to read the book, then I'll check it out at the library in *whatever* format it is in, rather than pay $14 for it.