How much should you be paid for an e-book? As with so many things - that depends. There is no concensus so far. There are two very different situations in which you may be negotiating e-book royalties.
If a book is produced only as an e-book and never on mashed-up dead trees, there are certain costs that will never arise. These are the costs relating to printing, shipping and warehousing books - and then, on a bad day, pulping remainders back into mashed-up trees. (There is no need to pulp the electrons if they don't sell the e-books. Indeed, pulping electrons would cause a cataclysmic disaster, so let's not suggest it.) But all the other costs still exist, at least if the book is done properly. The costs of commissioning, editing, design, layout, and producing the final files are the same (though design may be lower, with a drab page design and no glossy cover). If the e-book sells for a substantially lower price than a paper book, the publisher may offer exactly the same percentage royalty as for a paper book. This might seem unfair, but it's possible that the maths are correct. A paperback produced in a large print run may have a printing cost of only a 25p or so; if the e-book sells for around two thirds or half the price of a paper book, the publisher is reducing their risk but not their costs.
If a book is produced as a paper book and then also as an e-book, the situation is rather different. The publisher is aiming to break even (at least) on the paper sales, so any e-book sales are a bonus that cost next-to-nothing to generate. (To convert from a print-ready file to an e-book format is the work of minutes, and even with checking and so on it's only a few hours). If the publisher is offering you the same royalty for print and e-book in this case, you're being cheated. The Society of Authors recommends asking for a royalty in the region of 40%. If a book is in print anyway, it is entirely reasonable for the profits from the e-book to be shared pretty much equally between publisher and author as neither is doing any substantial work to sell the e-book, and neither is taking any extra risk.
Many publishers are sneaking low-royalty clauses into contracts while authors aren't looking. You may think that your book won't become an e-book - but it may do, or it may be used in some other electronic format which your publisher hasn't even thought of yet. You need to keep electronic rights, say explicitly in the contract that they are to be separately negotiated or, at the very least, licence these rights for a short period with a review after 18 months or so. Things are moving quickly - the deal that looks OK now might look atrocious in 12 months' time. And you might even want to produce your own e-book... but that's an entirely different post for another day.