You sure you're not talking about the RAM expansion cartridges for improved performance on some fighting games?
Cause I can show you even just on eBay that regular official memory cards can be found for 20-30 bucks, the Japanese ones even hovering between 10-15 bucks (although some of those look unofficial).
I can't speak to the import or third party cards, or newer reproduction cards that some ebay sellers might have. But the official Sega memory card in the US was the one that says "Backup" on the front. It just provides storage; no additional performance. Like this one here:
https://www.ebay.com/itm/Vintage-SEGA-S ... 3428100275?
That's the most recent sale on ebay, for $65 shipped. Prior to that, one sold for $30 but only because the seller listed it too low with Buy It Now. The others before that ended at $75, $70, $118 (with original box), $65, $64, $60, etc... There aren't many of the official cards sold in the past couple months. And I saw less than 10 of those currently for sale on ebay.
But again, my point wasn't really about the price. I was saying that the very few official cards listed on ebay, combined with the high price they sell for, is a good indication that it was not a common accessory that US Saturn owners bought. And Sega seemed to treat the internal memory as the main save method (after all, they called their memory card a "backup" card).