Is there a way to pull the customer's default price level into a grouped Results field for a transaction search?

The transaction search basically groups/sums sales order amounts by client. I then want to compute the approx number of hours that we should allocate to that work, based on (summed amount / client's default price level). For example, the client may have $1000 worth of services on an invoice, and their default price level (for services) may be $100/hr.

The following is an example of Formula (Numeric) field of Summary Type "sum" that returns an "Invalid Expression" error:

sum(case when  {type} = 'Sales Order'  then {amount} else 0 end)/{customer.pricelevel}