Perhaps something like "Save 68% off individual list prices with Baen's X Bundle"? Or if you want to make the amount of the discount more obvious, "6 books for $18 in Baen's X Bundle (regularly/individually $57)", adjusting the numbers as needed for each month.
I'll also note that for people who read Mercedes Lackey, her book in this bundle is not just an omnibus of her two former Baen Free Library story collections, but also adds several new previously-uncollected shorts (something that's not obvious from the catalogue blurb).
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