Hi Jason,
Notebooks are stored as items and will consume storage credits accordingly (you can find it detailed in this ArcGIS Online help doc topic: Understand credits—ArcGIS Online Help | Documentation).
For running notebooks there are no credits charged for running notebooks during the public beta period, but anything you do in the notebook that would ordinarily consume credits still does (for example, running analysis or accessing Premium Content from the Living Atlas). More information can be found here: https://community.esri.com/thread/239476-does-participating-in-the-beta-consume-credits We also invite the GeoNet community to give us there thoughts/ideas on credits relating to notebooks on that discussion thread.
After the beta period ends any notebooks that use and Advanced or Advanced with GPU Support notebook runtime will consume credits (you can choose if your notebook uses these or not and need Advanced Notebook Privileges to create notebooks of this runtime type). We will be releasing more information on specific credit costs per notebook runtime as we get further along in the beta.