Generally, IME, they sell ebooks for less than paperbooks but give the author a higher percentage (often around 40%).
So, paper book may be sold for £10 and you will get £1 (assuming 10%, lets make this easy) for every book sold.
Same book as an Ebook will be sold for £5 and you might get £2 from that.
Main difference between self and epub in terms of money is that a good publisher should be marketing your book to increase sales (as well as making it more saleable by editing and doing good cover design etc). So, in theory, you can get more despite the lower percent profit. So, in the above example:
Ebook self published (sold for £5) with 70% cut, author gets £3.50, sells 100 copies making total profit £350
Ebook published (sold for £5) with 40% cut, author gets £2 per copy, sells 200 copies for a profit of £400.
This is assuming that the publisher can market your book enough to get sales but from what I have seen most are better at it than a single author working alone.