Depends on a bunch of factors. A really quick look at 2024 printing services turns up quotes for as little as 16 cents per book (plus a $20 setup fee) to as high as $1.50 (with no setup), and all of those vary based on how many you're having printed at once. You'll also get more rejects, which may or may not affect total costs depending on your contract - some printers eat that cost, others don't. Cheaper than it was the last time I looked, but not by much. That's how much extra the publisher pays, so adjust for wholesale discounts if you want any kind of distribution and decide how much profit you want to make at the end of the day. $5 is probably way on the high side (I was being facetious), but a $2-3 markup for the buyer? Probably not implausible for smaller print runs - which doesn't excuse WotC, of course.
Maybe someone who's done a book with a ribbon bookmark can give a firm figure on what it cost them. I'm honestly curious what it runs these days, but don't feel like digging more definite info out of the printing marketplace.