Keep in mind that there's a point of diminishing returns when it comes to SD card speeds. The Steam Deck only supports Class 1 of the UHS (Ultra High Speed) specification. UHS class 3 cards are faster, but unless you are going to use them elsewhere or get a great deal on one, it might not be cost effective to purchase one to use in the Steam Deck.
I only mention this, becuase I'd hate to see someone spend a premium price for a UHS 3 SD card because of great benchmark results that won't be achieved in the Steam Deck.
I assume most people on this forum are not from the Netherlands, but I got a 512Gb (UHS Class 3) SD card for €59.95 last week. I use a Dutch price-watching website, and €60,- was my alert number Basically all Class 1 cards are the same price, so I figured, why not.