Define a mapping for "BENCHMARK COST" in 3 consecutive Query transforms.
Query transform #1: lookup(BENCHMARK, "BENCHMARK COST", null, 'PRE_LOAD_CACHE', YEAR, YEAR, MONTH, MONTH, "HOSTING CLASS ", "HOSTING CLASS ", REGION, REGION)
Query transform #2: decode("BENCHMARK COST" is not null, "BENCHMARK COST", lookup(BENCHMARK, "BENCHMARK COST", null, 'PRE_LOAD_CACHE', YEAR, YEAR, MONTH, MONTH, "HOSTING CLASS ", "HOSTING CLASS ", REGION, null))
Query transform #3: decode("BENCHMARK COST" is not null, "BENCHMARK COST", lookup("STANDARD CLASS COST", "BENCHMARK COST", null, 'PRE_LOAD_CACHE', YEAR, YEAR, "HOSTING CLASS ", "HOSTING CLASS "))