Your "the more you buy" analogy doesn't quite fit.
Its not the quantity of work but the *type* of work. Quick simple jobs are charged at a lower rate than more complex or difficult jobs. You could have six easier jobs on a ticket(all less than 1.0 hour, let's say) and never even enter into the part of the matrix where the "gouging" takes place. The customer will be paying less per hour than your posted "door rate", as most matrixes start *after* 1.0 hours or so. In fact, the best matrixes start tapering off after 8.0 hours or so.
Another way of looking at it:
Go to Walmart (or Safeway, or Home Depot) and there are always very good prices on some of the most popular items (Corn Flakes and Campbells' Chicken Noodle soup). On the rest they make a very tidy profit (New York strip steak and Entemanns pastries). You can buy as many boxes of corn flakes as you want and get the same bargain price. If you need New York steaks, you'll have to pony up.
Go to Hometown Chevy and there are very good prices on some of the most popular items (oil changes, wheel alignment, etc.) On the rest they make a very tidy profit (tranmission overhaul and driveability repairs). You can buy six competitive service items on a repair order and get the bargain price on all of them. If you want your tranny overhauled, you'll have to pony up

Cheers
DD