Calculate the most common cost

Try this:

First a measure to count rows.

NoRows = COUNTROWS(Data)

Then figure out the most common cost. The FIRSTNONBLANK deals with ties, without it you will get an error in the precense of the two (or more) cost values being most common.

MostCommonCost = 
FIRSTNONBLANK(
    TOPN(
        1, 
        VALUES(Data[Cost]), 
        RANKX(ALL(Data[Cost]),[NoRows],,ASC)
    ), 
    1
)

That should do it.