A company uses linear programming to decide on the production and sales budget that will maximise total contribution and profit for a financial period. The optimal solution involves using all available direct labour hours, for which the shadow price is $4.50 per hour, and machine hours, for which the shadow price is $3 per machine hour. Direct labour is paid $8 per hour. It the objective of the company is to maximise total contribution and profit in each period, how much should the company be willing to pay per hour to obtain aditional direct labour hours of production capacity?