Measure to build variable starting price

Hey Community

I'm working on a PBI report with data about cars and their prices.
My table consists of the following columns:

• YearMonth
• VehicleID
• Make
• Model
• FuelType
• BodyType
• Type (new car, used car...)
• km
• HP
• Age (m)
• Price 
• Date

  The dashboard viewer can filter the data using slicers (bold text in list)
  What I want to do now, is to build a measure that calculates an average start price (for the first month in the selected time period) for all the cars in the filtered set.

  Using this start price I want to show the development of the prices in percent over time (by month).

   

  I build a measure that calculates the start price:

oneStartPrice = CALCULATE(AVERAGE(One[Preis avg]), DATESBETWEEN(One[Date], DATE(YEAR([minDate]), MONTH([minDate]), DAY([minDate])), DATE(YEAR([eomMinDate]), MONTH([eomMinDate]), DAY([eomMinDate]))))

minDate = CALCULATE(MIN('Calendar'[Date]),ALLSELECTED('Calendar'[Date]))

but this measure is time-dependent and wont work (returns a blank) on other months than the minimum month selected.

is there a way i can make this measure to work for all the months? 
thx in advance, tanja