A 15TB 2.5" SSD (MZILT15THMLA) is ~$6000, or $12000 for the 30TB (MZILT30THMLA), both at $400/TB. 10 of these cards are $4500 for 10TB at $450/TB. If you pay that much on storage you'll probably want something that offers you more capacity, performance, and reliability then a multi-microSD setup.
[updated to correct pricing and calculation based on observation made by the fellow commenter below]
I'd never trust a single 15TB device though, so would need to consider the cost of multiples of them in my VFM calculations.
Then again even 1Tb devices make me nervous due to the amount of time it can take to rebuild your redundancy level after replacing iffy units, and might insist on straight mirrors (R10) rather than something more space efficient like R5 to try keep rebuild time down as low as possible.
I wonder what these are for ? Is it for devices with limited hardware (single 2.5" slot), or is it because it is sometimes the cheapest way to accelerate a system (hard to beleive) ?
[updated to correct pricing and calculation based on observation made by the fellow commenter below]