Having the backside printed in grayscale is free. You can pay a bit more and have it done in color if you prefer.
Instead of regular old business cards, I've been ordering postcards from Vistaprint to advertise my work. They're affordable and have always come out looking the way they should (e.g. the colors are true.) Here's the latest one that I ordered, front and back. The backside actually looks even better in person since the photo I took of it made the writing look crooked. I don't usually endorse companies in my posts, but Vistaprint is one that you should look into if you are thinking of promoting your work. Of course, they offer traditional business cards, too, along with a mess of items that you can have your image printed on.