SQL / Power Query recursive - From Startdate to 12 month filled timespan

On the CTE part, what SQL engine are you using? that is going to determine if recursion is supported.

If you want to use powerquery

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTIwVPBKzFMwMjAyBfIMjQyUYnWilYxAMuYKwakFIBkDsAxYwhgkYangn1wCkgAZYAbUEQsA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Project = _t, Startdate = _t, Costs = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Project", Int64.Type}, {"Startdate", type date}, {"Costs", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "months", each 
List.Generate(() => 0, each _ < 12, each _ + 1)),
    #"Expanded 12 months" = Table.ExpandListColumn(#"Added Custom", "months"),
    #"Added Custom1" = Table.AddColumn(#"Expanded 12 months", "Dates", each Date.AddMonths([Startdate], [months])),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "Partly Costs", each [Costs] / 12),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom2",{"Costs", "months"})
in
    #"Removed Columns"