I think a couple of options might help:
Each resource has 5 different rate tables. You could populate the rate tables with the differing rates per location.
In the Task Usage view, add the Rate Table field - change the rate table and Project will recalculate cost based upon the new rate table. (Fill down works to quickly change the rate table for multiple assignments).
You cannot change the value calculate in the Cost field - but you can create custom calculates in the spare Cost1, Cost2, etc fields.
I hope this helps.