Most E-Books are nearly as costly as books printed on paper. However, whereas pre-1924 classics are out of copyright and therefore inexpensive in print, many such classics and old books are free or nearly free in digital form. Most distributors of e-books have some free classics.
The HathiTrust is a Digital Library where you can read public domain books that were lent by Research University Libraries to be scanned by Google.
The Internet Archive has also begun a large book scanning project to preserve culture that is in the public domain.
Google Play Books has a large selection; try searching for “free classics.” (You will need to have a (free) Google Account to download these.)
Amazon also has a large number of free classics. Trying searching in books for “free classics kindle.” You will need to sign in to Amazon to receive free classics.
800 free eBooks from Open Culture.
Apple’s iTunes store also has free classics, though they are slightly harder to search for. In the “books” tab, select the “top free” list on the right, and then for classics, sort by release date and scroll down to find the classics.