How to do a running Sum by group in Power Query?

You can use this query (assuming you want to group on "BU"):

let
    Source = Table1,
    TableType = Value.Type(Table.AddColumn(Source, "Running Sum", each null, type number)),
    #"Grouped Rows" = Table.Group(Source, {"BU"}, {{"AllData", fnAddRunningSum, TableType}}),
    #"Expanded AllData" = Table.ExpandTableColumn(#"Grouped Rows", "AllData", {"Location", "Month", "Cost", "Running Sum"}, {"Location", "Month", "Cost", "Running Sum"})
in
    #"Expanded AllData"

With function fnAddRunningSum:

(MyTable as table) as table =>
let
    Source = Table.Buffer(MyTable),
    TableType = Value.Type(Table.AddColumn(Source, "Running Sum", each null, type number)),
    Cumulative = List.Skip(List.Accumulate(Source[Cost],{0},(cumulative,cost) => cumulative & {List.Last(cumulative) + cost})),
    AddedRunningSum = Table.FromColumns(Table.ToColumns(Source)&{Cumulative},TableType)
in
    AddedRunningSum