The Beachcomber Gallery (Mon.-Sat. 1000-1500), at the corner of the Ara Tapu and Makea Tinirau Road, is housed in a former London Missionary Society school building (1843). It's worth entering this museumlike gallery to peruse the lovely black pearl jewelry and other artworks on sale.
Moana Gems and Paka's Pearls , both in CITC Shopping Center, sell black pearls from Manihiki and Penrhyn with prices clearly markered.
Unfortunately, Bergman and Sons at Cooks Corner doesn't price their pearls, though it's still worth a look.
Farm Direct Pearls, in Harbor House opposite Avatiu Harbor, sells Manihiki black pearls with prices clearly marked.
Tokerau Jim (closed Sunday) at Matavera has a good selection of black pearls, including carved pearls, all clearly priced.