Between Seattle and St. Louis, 198. You generally have to stay over a Saturday night and book two weeks in advance. But you have until May 24th to buy your ticket and you can fly through September 10th. But wait, there's more.
Some deals are available on a four-day advance purchase basis.
But you must buy your ticket by May 16th and conclude travel by May 26th. Bid adieu to TWA by saving big on summer travel.