SQL weight multiplication
This is one way to accomplish this, but I agree with JNK in the comments that it will scale very poorly and perform the same:

    SELECT CASE WHEN ISNULL([PackWeight/Vol], '') = '' OR PATINDEX('%[a-Z]%', [PackWeight/Vol]) = 0 
THEN '' 
ELSE CAST(SUBSTRING([PackWeight/Vol], 1, PATINDEX('%[a-Z]%', [PackWeight/Vol]) - 1)  * [Units/Pk] AS VARCHAR(256)) + 
               SUBSTRING([PackWeight/Vol], PATINDEX('%[a-Z]%', [PackWeight/Vol]),  LEN([PackWeight/Vol]) + 1 - PATINDEX('%[a-Z]%', [PackWeight/Vol]))
    END
        FROM table