DAX Calculated Column to identify price deviation

The mode is the most frequently occurring value and DAX Patterns gives this general approach:

Mode :=
MINX (
    TOPN (
        1,
        ADDCOLUMNS (
            VALUES ( Data[Value] ),
            "Frequency", CALCULATE ( COUNT ( Data[Value] ) )
        ),
        [Frequency],
        0
    ),
    Data[Value]
)

This prior post gives further examples of how to apply this:
https://community.powerbi.com/t5/Desktop/Calculating-mode-of-a-measure/m-p/83116

Since you don't want the mode over the whole table (just the current product), you'll have to do a bit of additional filtering:

PROMO / OVERPRICE :=
VAR CurrProduct = Sales[Product]
VAR FreqTable =
    GROUPBY (
        FILTER ( Sales, Sales[Product] = CurrProduct ),
        Sales[Retail Price],
        "Frequency", SUMX ( CURRENTGROUP (), 1 )
    )
VAR Mode =
    MINX ( TOPN ( 1, FreqTable, [Frequency] ), Sales[Retail Price] )
RETURN
    SWITCH (
        TRUE (),
        Sales[Retail Price] < Mode, "PROMO",
        Sales[Retail Price] > Mode, "OVERPRICE"
    )