“Making a statement about a product that would discourage people from buying it constitutes speaking loshon hora as it could harm the livelihood of either the manufacturer or the storekeeper who sells it.” (Chofetz Chaim: A Lesson A Day by Rabbi Shimon Finkelman & Rabbi Yitzchak Berkowitz, p. 92)
Iacisrmma, it would seem to me that based on this, saying that store x sells an item cheaper than store y would be loshon hora since it will discourage people from buying at store y.