Final answer:
The difference in advertising costs per unit of output between the existing firm and the new entrant is $9.
Step-by-step explanation:
The difference in advertising costs per unit of output between the existing firm and the new entrant can be calculated by dividing the total advertising costs by the number of units of output sold. In this case, the existing firm spent $300,000 on advertising costs and sold 50,000 units of output, resulting in an advertising cost per unit of $6. The new entrant matched the advertising expenses at $300,000 but only sold 20,000 units of output, resulting in an advertising cost per unit of $15. The difference between these two costs is $15 - $6 = $9.