How to aggregate hour and cost rates by task

Hi Fusion community,

We can't use the native user or job role cost rate fields, so we went ahead and created a custom field on the user object to store, and hide, cost rates for each user.

I want to use Fusion to calculate a task's sum cost based on all of the hour entries against it and users cost rates.  We have multiple users logging hours to a task.  Each user has a different cost rate.  I've gotten as far as calculating the sum cost, per users on a task, but I'm having trouble figuring out how to aggreate these values into one Task Cost value.

Here's what I have:

I'm getting the correct CostbyUser for the task, but I'm wondering if setting this up as a variable might not be the way to go about it in order to them aggregate all of the CostbyUser values?

Thanks!
Nick