Most dive shops rely on sales of equipment to continue their existence. The margins on courses can be slim to non-existent in many areas, depending on number of customers, local regulations, insurance requirements, equipment/boat hire etc. Obviously it varies.
The problem is, everybody wants something for nothing. A friend of mine has a standard reply to people who ask for a discount on a dive course. He tells them: "sure - you phone your boss right now and tell him you'll take a 20% pay cut, and I'll give you a 20% discount on your Open Water Course." For a group of 8 people, then yes, group discounts might apply - but for 1 person, the margin might be close to nothing.
Many dive shops are struggling to stay in business and compete against the online stores. As one poster mentioned Wal-Mart - this isn't an industry where we can bulk-buy tshirts, mark them down and make everybody think they're getting a bargain. Most dive equipment is highly specialised, quality manufactured to high standards, often specified by government. By its nature it's not cheap - and your average LDS is maybe selling one or two BCDs a month...
Some LDS will be willing to give discounts on some items, and if you build up a friendly relationship and bring repeat business then it's almost guranteed. Consider also that walking into your LDS might be more expensive than shopping online, but the personal service, advice and the ability to try-before-you buy are often worth the extra cash. If your LDS isn't offering these things, then go somewhere else.
I'm pleased to see a comment like Rad Diver's above - it's exactly what I'm getting at. By building up a relationship with your LDS you're more likely to get discounts with the quality advice. Cheaper is not always better.
Safe shopping,
C.