Sometimes a card base can be as easy as a piece of printed vellum on top of a card layer. The samples below are from Magenta. These are all 12 x 12 sheets, at $1 each. The designs are lovely and simple enough that they will work for many occasion cards. All you're left to add is a layered salutation with perhaps a die-cut or two, and you have an instant card.
In the card on the left, we have cut the vellum longer than the card, so we could fold over at top and bottom to adhere to the inside of the front of the card so the adhesive doesn't show. We then cut a liner for the inside of the card.