Cost per donation is a useful metric, but it has some limitations that you should be aware of. First, cost per donation does not tell you anything about the quality or the value of the donations. For example, if you have a CPD of $10 for a campaign that generates 100 donations of $100 each, and a CPD of $5 for a campaign that generates 200 donations of $10 each, you might think that the second campaign is better. But in reality, the first campaign raises more money ($10,000 vs $2,000) and might have more loyal and engaged donors. Second, cost per donation does not account for the long-term impact or the lifetime value of the donors. For example, if you have a CPD of $10 for a campaign that attracts new donors who will donate again in the future, and a CPD of $5 for a campaign that only appeals to existing donors who will not donate again, you might think that the second campaign is better. But in reality, the first campaign has more potential to grow your donor base and increase your revenue over time. Third, cost per donation can vary depending on the type, size, and duration of the campaign, and the external factors that influence the donor behavior. For example, if you have a CPD of $10 for a campaign that lasts for a month and targets a large and diverse audience, and a CPD of $5 for a campaign that lasts for a week and targets a small and niche audience, you might think that the second campaign is better. But in reality, the first campaign might have more room for improvement and optimization, and the second campaign might have reached its saturation point.