I am not familiar with CPM. What features would you like to get in Power BI?
You can import required data from your data source to Power BI Desktop, then calculate budget using DAX in Power BI.
I remember seeing something similar with your requirement from a vendor's (poweronbi) showcase in last year Data insight Summit. Their tool is called Power Planner which has the ability to writeback to the database. Here's the demo:
what you suggest it is a known forced workaround to remedy to the absence of the write-back feature. Nowadays, each Microsoft on-memory tabular technology (e.g. SSAS Tabular, PowerPivot, Power BI) hasn't a such feature, but it is present for SSAS multidimensional.