Many customers have noticed their priority data consumption increase dramatically after installing a Nest Cam. The amount of data it consumes depends on your video quality setting described in this article: https://smarthomestarter.com/how-much-data-does-nest-cam-use/. Here are some estimates:
You can change your video quality setting on your Nest camera by following these instructions: Change Nest camera or doorbell video quality - Google Nest Help.