Castrol have always been Group III, besides their 0W40 which has been PAO, so I would say they price their 'synthetics' accordingly against Mobil 1. Trex101, we don't get GC down here. Aussies aren't keen on 30 weights.
To explain what I said before, Motul Turbolight is a semisynthetic, not even a Group III which makes it a ripoff at $50 a bottle. You can get Group IIIs such as EDGE 5W30 or Helix Ultra for the same price.