Final answer:
The seller's cost of capital would have to be 4% or higher for the discount to be cost-justified.
Step-by-step explanation:
In order for the discount to be cost justified, the seller's cost of capital would have to be equal to or higher than the effective rate of return the firm would invest at. Let's calculate the effective rate of return using the information provided:
- The customers receive a 5% discount if they pay within 10 days.
- Otherwise, they must pay in full within 45 days.
- The cost of financial capital for the firm is 9%.
- The firm can capture the 5% return to society.
Based on this information, the effective rate of return for the firm would be 4%.
Therefore, the seller's cost of capital would have to be 4% or higher for the discount to be cost-justified.