There are now official, legal complete omnibus collections of the 13 (including the complete short story collection)
Miss Marple and 5
Tommy & Tuppence books from HarperCollins (at least in Canada, linkage to Kobo) which were added in late 2013, at a price that works out to roughly $3-4 CAD per book, which is probably about as good as you're going to see unless HC runs another 99 cent sale on Christie again.
But there's no Poirot as in the OP request, probably because it's some 30-odd novels, IIRC. Though they still might at some point in the future once they're done welding* them all together.
* I wish that e-book bundles really were bundles, with a single purchase giving you a discounted access to the original individual books rather than a squished-together frankenfile containing the lot.
Some of us like to flip between more than one title at once, which is harder to do in an omnibus compilation, especially ones that consist of randomly selected vaguely-related genre-themed authors. And have all the cover art if the publishers have bothered to include it.