Alright - info on this is very sketchy, but I have a best-guess at why there's no price associated with ebooks through the Product API.
Amazon don't allow any of the usual Offer ResponseGroups, they cut that out a while back. Although the list price SHOULD be available, Kindle books don't seem to have one to start with - they have a Kindle price which isn't provided, as far as I can tell.
So far I've tried ItemSearch with a SearchIndex = KindleStore, ItemLookup by ASIN, and I've gone through most of the more likely ResponseGroups in order to find any price associated with a Kindle ebook, and there is no sign of one in any combination. At this point, I'm going to assume that ebook prices are not available at all through the API for unknown reasons at this time.
So the only way to get it is by using a scraper and risking Jeff Bezos' wrath.