You can buy 512gb cards on Amazon for $15.99 here…
You’re right that there’s plenty of cheaper Micro SD cards on Amazon, though I’d be reluctant to buy the cheapest large size card available. There’s a ton of fake cards out there that have much less memory than advertised (either no-name brands, or knock-off big name brands).
That’s not to say I’d trust a “Contixo” brand Micro SD card, which are certainly just cheap Chinese cards with the “Contixo” name stamped on them. It’s also pretty funny they’re selling cards up to 256GB here, when the tablets being sold on the same page only take cards up to 128GB.