averages

Here is one way to do it in the query editor.  To see how it works, just create a blank query, go to Advanced Editor, and replace the text there with the M code below.  Note that two steps of the same query are appended to get your desired result.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ11DUyMDJQ0lEyBGITpVgdFEEjIDZGFzQGYlN0QROwESBBU1QzMQSNwBhN0BhukSmqmWZKsbEA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, Product = _t, Cost = _t]),
    InitialTable = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Product", Int64.Type}, {"Cost", Int64.Type}}),
    Custom1 = InitialTable,
    #"Filtered Rows" = Table.SelectRows(Custom1, each ([Product] <> 4)),
    #"Grouped Rows" = Table.Group(#"Filtered Rows", {"Date"}, {{"Product", each List.Min([Product]), type nullable number}, {"Cost", each List.Average([Cost]), type nullable number}}),
    TableToAppend = Table.ReplaceValue(#"Grouped Rows",1,5,Replacer.ReplaceValue,{"Product"}),
    #"Appended Query" = Table.Combine({InitialTable, TableToAppend}),
    #"Sorted Rows" = Table.Sort(#"Appended Query",{{"Date", Order.Ascending}})
in
    #"Sorted Rows"

Regards,

Pat