Stone Wall Towers cost 300 stone both before and after the Extensive Fortifications Vizier point is sent. This is despite being a defensive buildings. Additionally, the cost to convert a gate remains at 30 stone, despite Stone Walls now costing 20 stone and Gates costing 40.
Additionally, since the last updates, Stone Wall segments cost 25 and Gates still cost 50 stone, so the difference is only 25, but the base cost of upgrading a stone wall segment remains at 30 stone.