There's no single price because most Nintendo DS models don't use SD cards. Only the Nintendo DSi and DSi XL have SD card slots (for DSiWare, photos, music). The original DS, DS Lite, and 3DS family use different media (Game Cards for games; 3DS uses SD/microSD).
Standard SD cards (up to 32 GB, SDHC) work. Prices in the UK (as of 2024) for reputable brands (SanDisk, Samsung, Kingston):
Speed Class: Class 10 / UHS-I (U1 or U3) is plenty; the DSi doesn't benefit from faster cards.
If you have a 3DS/2DS, you need a microSD card (often included with the console). For original DS/DS Lite, no card slot exists—games save to the cartridge itself.