Measure Help

Dears i would need your help for a measure : i have a table with sales by month , i created another table with dates which i linked many to one: than i created a formula to calculate prices ; Price = DIVIDE(sum('Price Evolution'[Amount]),sum('Price Evolution'[Pieces])) Than based on this i created the following: Price Base = CALCULATE([Price],filter('Price Evolution','Price Evolution'[Calender Year_Month]=[Previous Month] Where Previous Month = SELECTEDVALUE('Previous Month'[Previous Month]) so when i select a month from drop down it actively calculates price for that certain period. Than i want to show data by month as in below Pic with interaction broken to the drop down so it shows all months and what i would like is instead of Price by month to show the delta vs the price of the month i selected and PRice of the month. At this point if i use Price - Price Base it gives me 0 for all months Any ideas on this ? Thanks https://ibb.co/xDDkX26