I'm fairly certain that employees get a discount much like employees at any other non-dive store...likely at the discretion of the owner.
Instructors and DMs usually only have access to keyman discounts through the LDS with which they are affiliated. "Independent" dive professionals may be able to get steep discounts direct from the manufacturers if they ask the right person. Normally, this "keyman" discount is arranged with the manufacturer through the dive shop at a certain time of the year. Bear in mind that the instructor/DM can usually only get the discount if the LDS carries a particular brand.
So how much is the "keyman" discount? It varies according to manufacturer. I'm not a dive professional so I can't quote you hard numbers (and I'd be very surprised if any instructor would willingly post the numbers in an open Internet forum, lest the manufacturer get po'd and cancel access to the discount). However, I have it on good authority that the discounts can be substantial with certain brands. For Aqualung/Apeks/Suunto gear, for instance, the keyman price might be approx. 50% of MSRP. Yeah, 50%. Whites drysuits might be 50%-60% MSRP (with undergarments/accessories included at no extra cost). On the less generous end of the spectrum, Halcyon might only give a modest 10%-15% discount. You can use your imagination with other brands. On account of this practice, many instructors can afford to purchase the newest, greatest, top-of-the-line equipment (regs) every year. To offset their new purchases, they might sell off the gear they used in the previous year. Often they can sell their used gear for more than they initially paid. Eye-opening, huh?
This arrangement is beneficial to everyone involved. The dive professionals get to use the latest, greatest stuff. The manufacturers get free, effective advertising. When a student looks into buying scuba gear, he/she is going to look at what the cool instructors and DMs are using, right? There's an implied message being sent that if a person is serious about diving, then he/she will purchase the same top-of-the-line equipment that the instructors/DMs use.
Think of this the next time you're reading some of the posts about gear-buying on this board. The whole price:value ratio is completely skewed for dive professionals who get keyman pricing.