Valve didn't initially provide all the details fans were looking for with Steam Deck but as they revealed more, the better the deal seemed to be.
Steam Deck is basically a portable gaming PC which makes the platform extremely attractive and it appears that Valve is fully embracing the idea of an open system since not only do they allow installations of competing platforms but also modification of the hardware, to some extent.
When the system specs were updated, Valve revealed that all three versions of Steam Deck will have expandable storage, even the 64GB one. This means the users would get what is practically a steal at a low price point since the meagre 64GB can be expanded through NVMe SSD slots.
That said, the sockets are 2230 m.2, not 2280 m.2. This might sound like technobabble but what it basically means is that you will have a harder time finding an SSD that will fit Steam Deck's socket and on top of that, it will be more expensive.
The exact pricing is bound to shift in the coming months as Steam Deck users will undoubtedly start looking for more storage and let's not forget everyone's favourite wastes of oxygen - scalpers. We can already imagine scalpers price gouging people because they depleted the stocks and are trying to make a profit by marking the price up for regular gamers.
Keep in mind that Valve also noted the SSDs are not intended for end-user (that's you) replacement. In other words, if you're not proficient at tearing down similar devices and putting them back together, it would be highly advised to find a professional who can install the SSD for you.