Answer
The price of the trade must lie between each party's opportunity cost of producing the other good.
Work Step by Step
As an example, Ben can make 4 boats or pick 4 boxes of oranges in an hour. Olivia can make 2 boats or pick 10 boxes of oranges in an hour.
Ben's opportunity cost of making a boat is a box of oranges (since both items would take an hour to produce). His opportunity cost for a box of oranges is a boat (since both items would take an hour to produce).
Olivia's opportunity cost of making a boat is five boxes of oranges (since both items would take half of an hour to produce). Her opportunity cost for a box of oranges is a fifth of a boat (since both items would take six minutes to produce).
Ben has the comparative advantage in producing boats, and Olivia has the comparative advantage in producing oranges. If Ben was to trade Olivia a boat, Olivia could trade two boxes of oranges, and the trade would make use of both Ben's advantage and Olivia's advantage.
For the trade to make economic sense (and take advantage of their respective comparative advantages), Olivia would need to be willing to trade at least one box of oranges (Ben's opportunity cost of making a boat) but no more than five boxes of oranges (her opportunity cost of making a boat).
