Calculate the average profit of prom brushed steel products.
prom brushed steel refers to p_type = 'PROMO BRUSHED STEEL'; average profit = divide(sum(subtract(multiply(l_extendedprice, subtract(1, l_discount)), multiply(ps_supplycost, l_quantity))), count(ps_partkey))
SELECT SUM(T2.l_extendedprice * (1 - T2.l_discount) - T1.ps_supplycost * T2.l_quantity) / COUNT(T1.ps_partkey) FROM partsupp AS T1 INNER JOIN lineitem AS T2 ON T1.ps_suppkey = T2.l_suppkey INNER JOIN part AS T3 ON T1.ps_partkey = T3.p_partkey WHERE T3.p_type = 'PROMO BRUSHED STEEL'