Get value/price from one date earlier

Not the most elegant solution, but it seems to work.

Sort your table by Product Code and then Date.

Then add two index fields, one starting from 0 and the other from 1.

Merge as New with the same table, using the Product code and indexes to join (see image)

Then filter the Productcode column to get rill of null values, and remove the index columns.

let
    Source = Table.NestedJoin(Table2,{"Productcode", "Index"},Table2,{"Productcode", "Index.1"},"Table2",JoinKind.FullOuter),
    #"Expanded Table2" = Table.ExpandTableColumn(Source, "Table2", {"Price"}, {"Table2.Price"}),
    #"Filtered Rows" = Table.SelectRows(#"Expanded Table2", each ([Productcode] <> null)),
    #"Removed Other Columns" = Table.SelectColumns(#"Filtered Rows",{"Productcode", "Price", "Date", "Table2.Price"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Other Columns",{{"Table2.Price", "Previous Price"}})
in
    #"Renamed Columns"

Like I said, not particularly clever, but might help you get the job done.