We are having a heck of a time finding decent flights under $200. We've tried Cheapoair.com at $240. Cheapair.com was a little better, with flights starting at $220...but on an airline we do not want to fly. I've tried Expedia.com, Orbitz.com, Travelzoo.com and every traveling .com I can think of! No luck.
Prime time to purchase tickets is supposed to be 6 weeks before the flight in the middle of the week. I'll keep checking back until we have success. After all, you only have one chance in a lifetime to turn 21 on the Strip!