Answer:
The marginal cost is $200 that she needs to complete the task.
Explanation:
Giving the following information:
Cassie has just spent​ $800 purchasing,​ cleaning, and reconstructing an antique quilt which she expects to sell for​ $1,500 once she is finished. After having spent​ $800, Cassie discovers that she would need some special period fabric that would cost her​ $200 in material and time to complete the task. ​ Alternatively, she can sell the quilt​ "as is" now for​ $900.
The $800 is a sunk cost. Now we need to determine the marginal cost. The marginal cost is $200 that she needs to complete the task.