In order to facilitate easier data joining in several of our products, we have changed Facebook Ads' backend name from "Amount Spent" to "Cost". This is because Facebook was the only case where it was not labeled as cost, so we wanted to make it match every other case.
The display name in the field list will still be "Amount spent" but it may appear in the results as cost.
We apologize if this change has broken some formulas or references in your reporting, but it was a necessary adjustment to provide better data. Please update your references to use the new name.