Magstripe cards seem pretty easy to clone (though I've never done it), since reader/writers for them are relatively inexpensive. I've been planning to pick on up to play with for a while. They aren't currently 3D-printable, because of the magstripe, but a pack of blank cards only costs a few cents to a few dollars depending on the type and where you purchase them so it's not really an issue. In total, starting from scratch, duplicating a card would probably cost you around $30-$80 and wouldn't need much in the way of computer skills. You won't be able to double-spend the funds though, for the same reason that duplicating your house key doesn't give you an extra house; it's simply another way to access your funds.
Chip-and-pin cards however, as used in Europe, seem to be more difficult to clone. I haven't done much research on them though so I'm not certain about that.
I think it's also worth bringing up why people clone cards: it's not to clone their own, but to swipe someone else's and clone that. Getting the strip info, for example, could be done through social engineering without the person realizing what's happening. I guess the best way to equate it is to stealing the number off a credit card to buy things online (the goal is to use it without the other person knowing the info was even taken), with the difference being that you're creating a physical copy instead of just a textual one.
I think that doing this is highly risky, though, being that pretty much anything you'd use the credit card for is traced in one way or another.
