How do you normalize a table that is indirectly dependent on a has-many relationship?
You can create a table ProductPrice:

CREATE TABLE ProductPrice (
  ProductID int NOT NULL,
  ProductSetID int NOT NULL,
  StoreID int NOT NULL,
  Price numeric(10,2) NOT NULL,
  PRIMARY KEY (ProductID, ProductSetID, StoreID),
  FOREIGN KEY (ProductID, ProductSetID) REFERENCES ProductProductSet,
  FOREIGN KEY (StoreID, ProductSetID) REFERENCES Store
)


ProductProductSet beging the many-to-many relationship between Product and ProductSet.

These two foreign keys would ensure that the price refers to a store that is intended to have such product. Then you can create a query to find out which prices are missing:

SELECT s.*, p.*
FROM Product p
JOIN ProductProductSet pps ON (pps.ProductID = p.ProductID)
JOIN Store s ON (s.ProductSetID = pps.ProductSetID)
LEFT JOIN ProductPrice pp ON (pp.ProductID = p.ProductID AND pp.StoreID = s.StoreID)
WHERE pp.ProductID IS NULL