I haven't paid full mark-up price for a Group-31s in years.
I have always bought "seconds", "blems", or "scratch & dents", whatever they happen to be called in your locale, usually for about 1/3 the cost of first-lines.
I have had as good, if not better, luck with the "blems" as I ever had buying first-line batteries.
The catch usually is you ain't gonna just be able to waltz in the local chain store and get these deals.
You are gonna have to become acquainted with the battery delivery guy; Interstate is usually a good one.
As a general rule, first-line batteries have a "when new" code stamped somewhere around the top edge.
The battery delivery guy will service the display racks at all the usual places and, when the code shows a battery has not sold in six months, he will put it back on the truck and replace it with a fresh one; just like the bread delivery guy.
These out-of-date batteries have all identifying labels stripped from them, get "BLEM" or "SECOND" branded across the top, and get a generic size/group sticker, then are sold at a highly discounted price.
Ask around the local big truck repair shops and anyone who has a lot of trucks and equipment and I bet they are on first-name basis with the blemished battery guy.