There are huge different, in terms of price, between fresh water cultured pearl and natural sea water pearl (South Sea Pearl). Furthermore the size and shininess of the pearl surface also contributes to the price. If you have no experience in buying pearl jewellery, my recommendation is, go to any well know shop like Chow Sang Sang, Chow Tai Fook etc and you can get some so-so quality fresh water pearl jewellery. Since you budget is around US$100, you may need to move down the quality scale to those small jewellery shops in Mongkok. For your info, the price for low quality fresh water pearls (real not fake) are pretty low. By all means, you are buying those for your bridesmaids and why buying high quality stuff unless your pocket is very deep.
Of course, if you are aiming for goodies and price is not a concern, I would recommend Mikimoto.