Look at it this way, in a cyber-sense: If you compare a detailed online article equalling a "page" that averages around 750 words (250 words being very small, 500 is usually barely enough, while 750 words are normal and anything over a thousand words is being fairly elaborate), then figure up how many pages (not words) you think your eBook should have within its chapters (if it has any), and you'll have a better idea. Let's say you are going for a 50 page eBook with pages that equal a common article of, lets say, 750 words. Well, right there, you'd have a roughly estimated 37,500 word eBook. It makes sense, right?
Personally, I just say write the dang thing and if you can't get at least 10,000 words, go file it under the 'short story' department and may success still come your way; cheers!