The Gaugin pearl shop on Rangiroa is one of many pearl shops, however it is the one to which you will be directed if you are staying at the Hotel Kia Ora. The shop comes to the hotel and picks you up in their air conditioned van.
The Gaugin is special because they are local producers. They will explain the pearl-making process to you, and show you their workers busily processing oysters. The range of colors you can find there is amazing, and you can get much better prices than on Bora Bora or the other Society Islands. You can also find a large selection of "B" grade pearls (B+, B, B-) which may be perfectly acceptable to you (it is harder to find a large range of these on the other islands), as often their luster and color are perfect, and there are a few tiny imperfections that can be hidden when they are mounted.
What to pay: The price range is very large -- from about $20 all the way up to thousands of dollars for a single pearl. Most of the pearls are well below $500, however. I bought a lovely pair of pearls and had them mounted into a pendant on the spot for below $400.
If you have seen Pirates of the Caribbean, you may have thought that the curse of the Black Pearl was just a name right? No, in Tahiti, some of the pearls are actually blackish in appearance.
As all things are in Tahiti, you could re-mortgage your house to buy one (well OK, that is a bit of an exaggeration, especially if you buy a poor quality one), but almost everything in Tahiti is seriously expensive. People expect you to haggle, so try your luck in several places to see what price you can get.
As Tahiti is so expensive, it may be advisable to buy the pearl unmounted, and get it mounted when you get home. I didn't acyually buy one, but they are pretty to look at!
I didn't take a picture of a black pearl, so this picture is courtesy of Tahiti Tourisme.
What to pay: Probably down to your skill at haggling...