Well, apparently Signature received a large grant that subsidized ticket prices for their season. I assume that grant doesn't cover the production costs for an extention - hence tickets are being sold at normal prices.
The reason for the sudden increase is because the Signature's season is subsidized greatly by TimeWarner (I believe that is who is doing it) which makes all tickets for all shows during the regular runs of their shows only $15. But if the show extends it goes back to regular Signature prices because TW is only subsidizing the regular season. Hopefully TW will do this again and other companies will follow suit.