Pearls should be purchased for the enjoyment you have when wearing and owning them. One should not think of them as an investment. Most cultured pearls cannot be resold for anything close to the retail price one pays for them. Someone on the forum here described pearls as being similar to a new car, once it is driven off the lot it is considered used, and instantly worth much less.
No, I don't recommend Costco. They don't have knowledgeable sales people to help you, nor much to choose from, at least the Costcos in the Portland area.
You can spend some time reading here and learning more about pearls, looking at the photos and seeing what you like, going to jewelry stores to look at their stock. If you do this you will make much wiser purchases with your budget. There are a number of trustworthy online pearl merchants who post here. Check out links under their names in the posts.