For your question, I summarized the pricing rules below for your reference:
Azure Functions Premium plan provides the same features and scaling mechanism used on the Consumption plan, but the difference is we can set the Pre-Warmed instances.
Premium plan can avoid cold start by setting the Pre-Warmed instances, for example we can set 1 instance for Pre-Warmed(but smaller than minimum instances size which you set), then if the function app hasn't been requested for a long time, we just need to pay for the warm instance. The price is shown as below:
The EP1 plan is 3.5GB for each instance, so we can calculate the price.
While the function app is requested, then it will billed on a per second basis based on the number of vCPU-s and GB-s(based on the number of the instance which your function app uses on a per secnond).
Hope it would be helpful to your question~