Alieia Boat Company manufactures $10$ luxury yachts per month. A navigation system is included in each yacht. Alieia Boat manufactures the navigation system in $-$ house but is considering the possibility of outsourcing this function. At present, the variable cost per unit is $$300,$ and the fixed costs are $$40,000$ per month. If it outsources the security system, fixed costs could be reduced by half, and the vacant facilities could be rented out to earn $$2,000$ per month of rental income. What is the maximum contract cost that Alieia should pay for outsourcing?

A$.$ any cost lower than $$2,500$ per unit

B$.$ any cost lower than $$300$ per unit

C$.$ any cost lower than $$4,000$ per unit

D$.$ any cost lower than $$2,300$ per unit