The problem is that people will doubtless charge way more than is needed for this library. I can get a city library card for $12 / year, which lets me check out unlimited books, movies, CDs, magazines, use WiFi, etc. There are very few (if any) books that are not in the city library system. $12 gets you the ability to talk to a knowledgeable librarian (who is being paid a salary, remember), go to a number of nice buildings across the city, etc.
Given that the operating costs of a digital library are going to be far less than the cost of maintaining 17 branches across the city (building maintenance, salaries, book / media purchases, etc), you should be able to subscribe to an ebook library for a fraction of that. (Even if they wanted to charge the whole $12 / year and make a killing, even that would be fine). Fat chance of that happening, though... all these subscription services are far more than that.
Oh well, I guess I will just stay with my current B&M library for $12 / year (which, BTW, also has a huge number of ebooks available, plus even lends ebook readers out IIRC).
Cheers