What are some good portraiture lenses for the Nikon mount at an affordable price?

I'm hoping to get a strobe, a ringflash adapter and a lens for my birthday, but I need help with choosing a lens, as they are important. I'm looking for something that's good for portraits and has a shallow depth of field.

Added (1). Nikon d3100

The 50mm f/1.4 is superb. Get the newer AF-S version so it will auto-focus with your camera. $440.
http://www.bhphotovideo.com/..._1_4G.html

If that's too much money, look at the 50mm f/1.8 which is pretty darn close.
$220 = half the money, but 98% the quality for portraits.
http://www.bhphotovideo.com/..._1_8G.html