How do you calculate Cost to Retain/Cost to Support for your Customers?

I would love to be able to calculate Cost to Retain  compare to CAC and calculate margin.

In the past I have used the more traditional hours from support, CS, customer marketing, training and renewals * average cost per hour (team expense) = total cost.  While this works, there is either a lot of overhead to track or there is so much generalization the metric isn't super accurate.