There are several locations around the whole Resort where you can pick-a-pearl, but my favorite one (and it's not any more expensive) is inside the Japan pavilion in Mitsukoshi.
They do a whole ceremony and make it a wonderful occasion.
Getting the pearl is about $20, and there are settings in both silver and gold. I think we saw prices ranging from another $20 up into the hundreds of dollars for the setting and necklace. (You can also just buy a cage for the pearl and then string it onto a piece of ribbon at home.)
This past September we paid $17/pearl from Epcot Japan-we do this everytime we go to WDW-pearls avg 6-71/2mm size and sometimes different colors or even twin pearls! there are alot to choose from on necklace-rings-pendants-etc avg $20 on up and they do it all on site
I did pick-a-pearl on my last visit to WDW at the Janan location. I got 2 picks for my DGDs...one white and one black. I purchased "Cinderella pearl cages" on eBay prior to my trip and just inserted the pearls into the cages when I got home. It was fun with a little ceremony and we took pictures of the whole procedure.....I plan on doing it again on my next trip.
We did this last trip, and got a nice pearl. DD will wear it for her wedding. The cage and the chain ran avout $40 dollars in addition to the cost of the pearl. Took about 90 minutes. The girl at the counter was great, and put on a very enthusiastic show. Was great fun.