After being on both sides of the counter, I feel I am qualified to answer the question objectively. First most dive equipment is marked up 100% of cost. this is a 50% profit margin. Don't get the two confused. Smaller ticket items may be more. The ability to get a discounted price usually depends on the local competition. A dive shop typically needs at least a 30% profit margin to survive and during the winter season in many areas the sales and business in general falls off a lot. Having said that there are companies who's dealer cost is less than 50% profit margin (cough) ikelite (cough)...you will never see their products sold less that 10% off retail because there is not enough margin for the dealer.
Now as a customer you are probably saying to yourself "what a ripoff, they are making a fortune off me." This is not really true and you have ot remember what kind of retail business you are dealing with. As a general rule, a business with high volume can sell at a lower profit margin and a low volume business must sell at a higher profit margin. The dive business is a very low volume sales market compared to a grocery store volume and very low markup around 1%-3%. (don't hold me to this-these numbers are old but they are still close).
There are other factors to consider. A dive store is not making much if any profit on your scuba class. They are hoping you will buy equipment, hence the need for mask, fins, and snorkel for o/w training. As a side note-instructors make little money teaching scuba but thats another story.
If you think the store is making a killing ask yourself a few questions. How much are they paying a square foot? You would probably be suprised that retail space rent goes for especially in a strip mall or high traffic street.
The classes, trips, and training are all designed to produce potential sales and that is the only real profit the store can rely on.
A smart dive store should treat a regular customer fairly and if you are looking for high ticket items, like a set of gear, you do have room to negotiate with a dive store and it is up to you and the store to come to terms if you want a deal. There is more to it but this should give you an idea and ask your self this, if you don't support the store and it goes out of business, how will it affect your diving?