As posted on Flyertalk, Washington-Dulles to Lima, Peru is running ~ $220 all-in, bookable any time through the end of the schedule (March, 2010). Use Kayak and enter IAD as departure city and LIM as arrival city to find it quickly.
You can get the price under $200 by flying United, Dulles – Mexico City and then LanPeru down to Lima. That fare, though, is only valid through June 17, and a Saturday stay is required.