There are some variables in profit margins for stereo equipment. It's not always 40%, sometimes it is more depending on the product and the arrangement between the manufacturer and dealer. Cables and other accessaries are almost always 50%. Mark up on cartridges are off the charts. I expect a discount when buying new unless I perceive the retail price to be a bargain.
Do you really believe cars are being sold at dealer cost? When buying a car you can't trust the salesman for little things, so why would you believe they are selling you a new car for what they paid for it? I see dealer invoice as just another fictitious figure.
It works the same way on car parts from the dealer. They are charging you double. My mechanic charges me half of what the dealer charges for OEM parts and everybody is still making money.
Some make money in quantity and others want to make it all at once. You can sell thousands of $89 DVD players, two $40,000 amplifiers or one set of $150,000 speakers.