Farmer Brian has 3 acres of land which he farms efficiently. Each acre can support 20 apple trees. However the 3 acres differ in their ability to support orange trees. He can grow 40 orange trees on the best land, 20 orange trees on the ok land, and 10 orange trees on the bad land. If he initially has his land split between apples and oranges (1.5 acres growing apple trees and 1.5 acres growing orange trees), what would be the opportunity cost of growing another apple tree?