Hi HomeschoolMama,
We fly to Disney from SAT about twice a year and I have never paid more than $325 round trip and the average is around $220 round trip. If your trip is scheduled for before Jan. 11th, are you looking at flights on Southwest? If your trip is scheduled for after Jan. 11th, then the fares might be higher right now because Southwest has not opened their schedule beyond Jan. 11th. Also, just an FYI - Southwest offers a nonstop flight between SAT and MCO. I don't know how old your fellow travelers are, but the nonstop sure makes it easier with small children.
If you are open to renting a car rather than using Magical Express and you don't see a decrease in the SAT to MCO fares, I would check airfare from SAT to TPA (Tampa) as well. The drive to Disney is not bad at all and the Tampa airport is much easier to navigate than MCO.
Either way, I highly recommend that you wait to book your air until Southwest opens up the schedule for the time you plan to fly. Feel free to PM me if you have additional questions.
-Kristi