A small custom built die might only cost you between $40-$80 from a reasonable die cutter. Even with your printer marking that up and the extra bindery time, a set of 1000 4-color custom die cut business cards will only run you about $400. If you have the budget for it, a die cut business card can be an extremely nice touch and the “little something” that sets yours apart.
Check out a bunch of inspirational business cards below that take advantage of die cuts. Most of these were taken from two great posts at Creative Bits as well as the incredible Daily Poetics Flickr set.
Also remember that die lines are not the only way to alter the shape of your card. Most printers have binderies that can do many other things like straight cuts, hole punches, and even rounded corners. These things are even cheaper that custom dies, so they are good to keep in mind when designing as well.