If you can do without the zoom, look at the 85mm f1.8 for under $500. But if you can move closer, the 50mm f1.8 is a bargain, and a very good lens.
Can you get the framing you want keeping your 55-200 at 55mm? If so, you'll be happy with the 50mm, and won't be spending too much to get it. (There's also a 50mm f1.4, for more money, but if you're price-conscious, the f1.8 will do fine.)