Is this normal for U.S ecpm?

It's recommended to wait for about 5000 impressions - it takes some time for our targeting algorithm to get an understanding on your player base before the provided ads are more relevant for each user. At around 5000 impressions you should have enough data points for analysing the game's monetisation potential better. Before that the numbers are too volatile for drawing any trend conclusions.

If you have any concerns about your monetisation design - feel free to reach the support for best practices by sending an e-mail to unityads-support@unity3d.com

We can then take a look and offer tips and best practices to ensure your monetisation design is ready for more clients.