We only do business cards if it's an existing client and we do all of their other work.
Our business model is that we do everything. We create the brand, come up with a name if they don't have one, create the designs for billboards, wall scapes, banners, cars, t-shirts, business card, we make the signs for the company, the flags, posters, flyers, packaging (labels for products etc), web sites, SEO...in one word - everything. For all their design needs they come to us and it's not an issue for them anymore. We will usually charge $10-20k for this but from there on it's a no headache deal for them, we take care of it all but they do all their print and signage with us. For these types of clients business cards tend to be on special paper with special features like holograms and the like and they of course have their price. The design of the business card is covered in the original deal and if they insist on cheapo business cards we usually try to persuade them to do it right, if they are insistant on doing the normal cheap paper ones, we do a nice design for them and tell them not to tell anyone that we made them...at that point they usually relent and say "fine, do it right".
If some random person in the universe comes knocking on the door and asks for a business card, we just tell them that we don't do those.