UPDATE
I received a reply from this QE instructor:
Paul Lorenzen
MAI, CPM, CSM, CCIM (Retired)
AQB Certified USPAP Instructor
He indicated that it is acceptable to use cost based adjustments in the sales grid as long as the adjustment includes entrepreneurial incentive (obtained via Market Participant Interview or Contingent Value Method). Apparently the addition of the entrepreneurial incentive component conveys 'market derived' status to the adjustment. He included a sample spreadsheet in his reply to provide an example, along with a thorough explanation. I think he assumed I was asking about a fix & flip assignment, but the as-is portion of his explanation seems to address the original issue (cost adjustments in sales grid).
Thanks again to everyone who replied -