In a sense, a sort of agree with our esteemed "brownivore," although I'm not quite as cutthroat in my attitude about it. I'd also slightly modify his quip and posit that "a fool and his money are lucky enough to have been together in the first place."

The free market capitalist system sort of depends on sellers being able to set their own price, and buyers using their intelligence to find the value. The beauty of the system (usually) is that the existence of competition among all the sellers trying to capture sales will result in the industry as a whole tending to offer value. Unfortunately, with an industry that is wholly comprised of "one of a kind" items like diamonds, this system sees some strain and requires more vigilance on the part of the buyer to determine fair value.
While I found in my search that I truly enjoyed the learning and researching, I realize that researching a diamond purchase is often not a fun thing for people to do, but it's sort of a necessary evil in the free market system on which so many economies are based. This is where forums like this come in, and where the Internet as a place to learn and compare really shines. Diamond databases are plentiful, and a lot of the information required to make an informed decision is right at your fingertips, without the need to see a single Rap sheet.
I found in my search that Internet dealers come in all varieties, from expensive and uninformative, to value-oriented and comprehensive. The good vendors give loads of information that can be used for everything from actually buying a diamond online, to using prices and specs gleaned from the Internet as tools in negotiating a price with a local dealer. The research might not be fun for many, but it typically pays off in the end.
Although the Rapaport Report is a commonly used reference, it's worth noting that it doesn't actually "set the price" in the marketplace, but simply attempts to
describe the market it covers. Also, since the Rap sheet exists
for the industry (not the consumer), it is understandably geared toward the dealer and is designed to some extent to offer them a way to make their prices look better than "going market rates." You might think of it as akin to the "manufacturers suggested retail price" of the diamond business (a value that usually has no bearing on the "street price" of most merchandise). I realize this analogy is an over-simplification, but it'll do for our purposes. I've seen dealers, upon being asked what a certain type of special-order diamond would be, grab their Rap sheet and flip through to find a price, but in reality this is not remotely related to what they might actually pay their cutter/distributor, so isn't the "final word" on the issue.
Unfortunately, since the industry does in fact deal exclusively in "one of a kind" items like diamonds, it would be very difficult for any resource to be comprehensive in rating
every conceivable combination of physical attributes possible in diamonds, so I'm afraid the Rap sheet price will never be a good indicator of the "real value" of a specific diamond, but it is still useful within the industry as a rough guide.
I personally didn't mind not knowing the Rap sheet price for diamonds in my criteria when I was shopping. I just canvassed every source of information and pricing I could find, compiled a fairly huge list of prices for stone within my criteria, and used those numbers as a guide for finding a "fair price" for my diamond. In the end, I think I did pretty well, and don't regret having not had the Rap sheet as a guide. It can be daunting, but with so many companies vying for consumer dollars there are definitely bargains to be found, and the Rap sheet isn't required to find them...
-Tim