The highly-inflated asking prices for the Scantenna ST2 on eBay have been dropping somewhat because the antennas are not currently selling. There are curently 2 listings on eBay for the ST2 antenna and both have a "Buy It Now" price and a place to "Make Offer." You might want to consider making an offer that you think is reasonable to you and you might just get lucky and have the offer accepted.
1. Scantenna ST2 currently listed for "Buy It Now" for $251.56 (was previously listed for $295.95):
ANTENNACRAFT SCANTENNA ST2 NEW IN UNOPENED BOX 30-1300MHz VHF/UHF. | eBay
2. Scantenna ST2 currently listed for "Buy It Now" for $229.00 (was previously listed for $299.00):
Antennacraft ST-2 (ST2) RARE scanner antenna, NEW in box! | eBay
The other antenna that you might want to look for would be the Channel Master Monitenna 5094/5094A which has a very similar design to the Scantenna ST2. The 5094 covered 4 bands (VHF-Low, VHF-High, UHF and UHF-T) while the 5094A covered 5 bands (added 800-900 MHz coverage to the original 5094 model). I just recently saw a couple of these antennas being sold (for $44 each) brand-new in the factory-sealed boxes which is pretty amazing considering that from what I read the antennas were discontinued by Channel Master around 2000.