The Halacha of Ona'a is that if an item was sold for more than 1/6 difference in real value the buyer and or seller can cancel the sale. How is Ona'a figured on for example fresh salmon? There are stores that sell it for as high as $12.99 lb and stores that sell it for as low as $6.99 lb. Is Ona'a 1/6 less than $6.99 and 1/6 more than $12.99?
Heard from Rabbi Herschel Welcher:
The problem with ona'a is only when fooling the buyer by making them think that your vastly-inflated price is the normal going price for this item. (Or vice versa.) But if you take what is reasonably known to be a 99-cent product and are selling it in some incredibly convenient location for $2, that's not ona'a.
So a given store's price could have to do with quality, location, convenience, all sorts of other factors. What you can't do is tell a customer "yes it seems expensive, but any grocery store around here sells the exact same product for this price" if that's not true.
(Specifically with regards to salmon, whether you get wild-caught or farmed, Atlantic or Pacific, whether it's product of China, etc. -- can make a huge difference. Not every pound of salmon is the same!)
Are you getting the same thing from both stores? Seems to me that the wide price disparity is due to difference in quality, either of service or product, or some other factor. Absent any material difference, there's no question that the higher-priced store is ripping you off.