In terms of grading, the GIA has not set any pearl grade standards. The most acceptable is A-AAA. But as I am sure you have found, this is not only subjective, but many retailers add additional grades to their mix. GIA does not assign a letter grade to any of the appraisals they prepare, and they do not assign an approximate retail value as well. GIA graduate appraisers do this service. However, it is again subjective unlike diamonds and other gemstones.
When comparing our pearls to another store with similar standards, but with an additional grading system, our ‘AA+’ should approximate another’s ‘AAA’. But only IF this seller carries high-quality Akoya pearls. This is important because if this seller does not have access to top-quality pearls, a low grade, albeit their best, may be categorized as ‘AAA’, even though it is nowhere near the best grade available.
Unfortunately when purchasing pearls either online or in a store you have to rely upon the honesty of the seller to know exactly what you are going to get. I would suggest asking very direct questions, and have them explain their grading as explicitly as I have explained above. What I typically tell undecided customers is to purchase a strand from us (we offer a 90-day money back guarantee) and then compare the strand to any others personally.
This is recommended reading:
http://www.pearl-guide.com/pearl-grading.shtml