I've given you some statistics and you're probably thinking that you can pitch your book to HarperCollins or any other book publisher and get your 25%. Uhm, not necessarily. Besides likely not being the most popular person on the planet, as most writers aren't, having your book published by a very well-known publisher is difficult. Many smaller publishers offer nowhere near that percentage of royalty. Think long-term if you plan to make a career out of book-writing and try to be strategic in your actions. Remember, if you want to make money from this writing thing, you need to view it as a business. Manage your expectations before you even get an offer and know that a foot in the door is important if you plan to publish numerous books continuously. Yes, I am telling you that I cannot say how much is acceptable.
Find out more about the established prices of ebooks and what the standard digital royalty rates are internationally by watching the video below. The technical difficulties in the video don't make it an easy watch but it certainly is informative.
If you don't know what royalties are, it's pretty much what the publisher is paying you to use your writing. Simple, right? Not necessarily. Always try to go over your contract with a lawyer before signing on the contract line.