How to calculate running total from events in progress?

Power BI
2

Create a new measure

Measure = 
SUMX (
    SUMMARIZECOLUMNS ( 'Calendar'[Year], 'Calendar'[Month] ),
    [3CostsActiveEvents]
)

Or

3CostsActiveEvents = 
SUMX (
    SUMMARIZECOLUMNS ( 'Calendar'[Year], 'Calendar'[Month] ),
    CALCULATE (
        VAR EndDatePerVisual =
            MAX ( 'Calendar'[DateKey] )
        VAR StartDatePerVisual =
            MIN ( 'Calendar'[DateKey] )
        VAR RESULT =
            CALCULATE (
                [Costspermonth],
                REMOVEFILTERS ( 'Calendar' ),
                FactsTable[StartDate] <= EndDatePerVisual,
                FactsTable[EndDate] > StartDatePerVisual
                    || ISBLANK ( FactsTable[EndDate] )
            )
        RETURN
            RESULT
    )
)