There are certain fixed costs which remain the same no matter the format: you have to (or at least should!) pay the authors, editors, proofreaders, artists, and cover designers the same amount no matter the form because they're doing pretty much the same work. And while ebooks don't have typesetting/layout processes identical to physical books, they still have them. And if you don't want the ebooks to look like ass -- something that a lot of big publishers clearly don't care about yet, unfortunately -- it's not going to take all that much less time, particularly if you have to prepare them in multiple formats; you have to do a "final proof," ideally on the actual devices you're targeting.

In practice, all of the costs above are generally much more of the production cost than the actual printing is. The biggest savings you have with an ebook isn't from not paying the printer, it's from not paying the distributor. But there's still a catch there: while you're not giving 50% or more of the cover price to the distributor the way you would if you were trying to get the book into Barnes & Noble or what have you, you're still giving 30% or more of the cover price to the retailer. (And you have to pay attention to the "or more"; you may be paying a middleman like Lulu to get you into some of the online stores, or you may be paying Amazon's bogus "download charges" in addition to their stated 30% cut. Assuming your sale qualifies for their 30% cut bracket, rather than one of the higher ones.)