This question was raised in the ‘Want to maximise your product design? See how a custom ASIC can help?' webinar, view all the questions in the round-up blog post.
Custom SoCs can save a significant amount compared to discrete components. The savings come about when a number of discrete analog components are integrated with the CPU. It is this integration that can save on component costs, and it is typically the number and the nature of the analog components that are integrated that have the biggest impact on the savings.
As each chip is unique it is difficult to give a precise volume, but a partner of ARM often suggests that it is worth looking at a custom SoC volume is around the 300k mark.