We've been doing STL for the past few years. We always vacation in Disney in Sept....I consider anything under $200 RT to be wonderful (seems like it costs that much to go anywhere from STL). We skipped last year, but in 2009, we paid $84 each way (plus fees and all).
Having said all of that....who knows what the fuel prices, etc are doing to this year's prices. I've been checking regularly on prices for Feb/Mar/April (since they usually seem to compare well to Sept prices). Right now, there are flights from STL-MCO for as low as $79 (depending on what days you fly, of course), though $109 is more prevalent.
We booked our flight down this year when they release them at $155 one way, hoping like crazy they go down and we can use the credit to buy the return trip. Today they went down to $135. I don't think I'm crazy for expecting them to get to $109. If they go lower, I will be thrilled. For the return in Oct, I'm going to hold out for $109 (we are returning on a Monday). Wish us both luck