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

Ok.

This is a modified fnAddRunningSum - nothing really important, just made the values column name a variable to untie the internal data structure to the main table from the computation logic in the function: 

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

This is the main table and the call to the function above using your sample data:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WKlTSUUoEYkMDpVgdBBeVZ4xPEsJLxsozwSdpik0yBdVKGNcMlYuNB3JPEgbPBJVrhE1tMqraZKyS5hiSsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Est = _t, CICL = _t, ve = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Est", type text}, {"CICL", type text}, {"ve", type number}}),
    
    TableType = Value.Type(Table.AddColumn(#"Changed Type", "Running Sum", each null, type number)),
    #"Grouped Rows" = Table.Group(Source, {"Est", "CICL"}, {{"AllData", each fnAddRunningSum (_, "ve"), TableType}}),
    #"Expanded AllData" = Table.ExpandTableColumn(#"Grouped Rows", "AllData", {"ve", "Running Sum"}, {"ve", "Running Sum"})
in #"Expanded AllData"

 Please note that the function call has changed slightly as we added the second parameter (value column name) to the function which calculates running totals.

This is the output of the main table/query:

EstCICLveRunning Sum

Please let me know if you have any questions.

Kind regards,

JB