Conditional query between tables
If I have interpreted correctly your question, here is a query that produces the result given the storeid X:

SELECT codproduct, 
       product_name, 
       COALESCE(l.price1, p.price1) AS price1, 
       COALESCE(l.price2, p.price2) AS price2
FROM Product p LEFT JOIN PriceList l ON p.codproduct = l.codproduct
WHERE storeid = X