There are several reasons why the cost per email might be higher. Firstly, if you're using a premium email marketing service, the pricing often scales with the number of features. For instance, advanced features like automation, detailed analytics, and segmentation can significantly increase the cost.
Another factor could be the quality of your email list. High-quality, targeted lists often come at a higher price, but they can also yield better results. Lastly, the frequency and volume of emails sent can also impact the cost.