-32GB can cost up to $100 (depending on the brand).
-Class # helps too, usually for that size Class 8/10 is nice.
-Adata, Sandisk are good brands, Adata is affordable too.
-Format any SD card using the SD Formatter that can be downloaded around before use.
-There are no retail games digitally yet, but around you can see that many are under 1GB (biggest 3DS-carts are 4GB but digital release won't have some of the extra junk retail releases have-).
Oh! and try not to buy 32GB cards on eBay or weird online just because they're cheap. There are many selling fake 32GB cards and can end damaging SD reader/slots.
EDIT:
Since the up to $100 is quoted so much....
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
As I said, is "up to". There are lower prices, there are deals, etc. but I chose to said it like that, because it also helps for people when looking and comparing prices; than by starting at the lowest price. There are many places that sell "cheap 32GB cards"; that are not the best for games.
There are deals for trusted places and good prices posted below.