Answer:
not particularly good, but within an order of magnitude
Explanation:
Cost per pair is 800/24 = 100/3 ≈ 33. So 28 is off by more than 5, about 16% of the actual value.
For some purposes, an estimate within 16% would be "good." For other purposes, not so good. In many engineering applications, an estimate within 10% is "good enough." In retail applications where profit margins are less than 5%, an estimate that is off by 16% can make the difference between profit and loss.
The estimate is not particularly good, but may be adequate.