Looks like Sandisk broke a major barrier. It is producing a 400GB microSD card. I'm not sure how much it costs. But, I imagine that other memory card makers won't be that far behind.
https://www.extremetech.com/computing/254891-sandisk-breaks-storage-record-new-400gb-microsd-card
https://www.extremetech.com/computing/254891-sandisk-breaks-storage-record-new-400gb-microsd-card