The plot sickens

I love looking for patterns and solving puzzles. Here's one I haven't cracked, yet. Check out this list of new teen releases, sorted by price. (I assume the list changes over time, but the pattern should still hold.) The first 61 of them (except, currently, for the first one, which doesn't have a price listed) are all offered for discounts of 70% to 88%. Each one has an ISBN starting with 9781615 or 9781616. Each is also available in the same format for a regular price with a different...