By the time something hits omnibus editions it's into a later phase of its life cycle and the publisher is just trying to mop up the last bits of profit to be found from a series. Fair enough.
My assumption has alway been that an omnibus doesn't really cost much more to make per unit than a single book, and all of the editing other costs have already been recouped out of the individual book sales. So an omnibus should cost a little more than regular book, but not a whole lot.
What's the point of an omnibus ebook? Really, by the time they're selling a 3 novel omnibus in paperback for $9.99, then the individual ebooks should be selling for about $3 each. Anything more is just gouging.
Personally, I think gouging on e-content is really dangerous in this day and age. It is so easy for people to get the content for free, and if they're pissed off enough they'll start to tell themselves that they almost have an obligation to do so in order to punish the gouging publishers.
You can't unring the bell on this. eBook sales are starting to become something that publishers can't just turn their backs on, and all it takes is one person to buy the content at full price, strip the DRM and upload it and it's over. So they'll need to keep public opinion on their side. Gouging isn't the way to do that.