I'm guessing you have one record per workshop, with a repeating field (not usually Best Practice in this scenario...) with various units of supplies costs. You can create a calculation field that adds all of those unit costs.
Presumably you have a field in your Attendance Table that links to this Workshop record, so you could include a Count calculation, using the relationship from the Workshop Table to the Attendance Table, to count an always-filled field, such as the AttendanceID, and multiply the total of the costs by that.
Unit Costs (your repeating field)
Total $7.00 <- calculation in the Workshop Table
Workshop Total Costs = Total Unit Costs x Count (Workshop_Attendance::WorkshopID )
Sum ( RepeatingField )
is the syntax you can use to get the total of all values entered into a repeating field.
Sum ( RelatedTable::Field )
also computes the total, but uses a related table for a set of values--such as your work shop expenses list, and it is a much more flexible approach than using a repeating field.
I'd still go with the related table myself. Keep in mind that a repeating field limits you to a specific maximum number of items in your list where a related table has no such limit. You get a new workshop with a very large list of items and you can end up having to redesign both your database and your layouts where you may not have to make any change at all when you use a related table.
That won't be a problem. The workshops are designed to have as few supplies as possible because they have to be lugged from school to school. And in most cases, this feature wouldn't even be used.