Fran,
AmFlyer is right about checking price guides. Price & Rarity Guide S Gauge 1946-2011, 2012 Edition by Robert Brubeck lists the set at $4,500. The condition and how complete your set is (boxes, paperwork, etc.) will determine a fair starting price.
Circus sets seem to be popular given all the reproductions that are available. A factory set should bring a pretty good price.
Your local Flyer guys should be very helpful.
Good luck,
Rich