Hi rglezmatl_cr thanks for your questions:
1. Insights refresh is charged on a per refresh basis. We default the refresh to "weekly", which means customers should be charged for about 4 refreshes in a month. If customers pick "Monthly" refresh cycle, then they would be charged for 1 refresh in a month.
2. Data Estate Insights looks for any changes in the Datamap since the last refresh. This could be the addition of more assets and sources or annotations on your assets. All insights refreshes are on incremental changes only.
In the last few months we have been diligently working to reduce customer's spend on deriving insights out of their DataMap. Please read the following blog to learn how we have passed performance and cost benefits to customers!
Do more with less using Microsoft Purview Data Estate Insights - Microsoft Community Hub