The Dolmio Express pouches of Tomato and Basil, or Tomato and Chilli sauces are 4.5 syns.
The Weight Watchers sauces are anything from 5.5 syns per jar (arrabiatta) to 6.5 for Mozzarella and Rocket, and 8 for creamy mushroom.
To be honest, save the syns and just use the passatta but add lots of herbs and garlic instead of having it plain. But those were the lowest I could find