Personally, I am dying to get the Freshadama set at Pearl Paradise... for $375 you get a necklace, bracelet, and earrings of the highest quality. I have an akoya necklace and earrings, but I am worried to wear akoya too much for fear of ruining them. Once I own some freshwaters, I'll be able to wear them as frequently as I like without worrying. So.... if you decide against the hanadamas or just if you want to get your girlfriend something she can wear on a regular basis and not ruin, I would get her some Freshadamas. If you still want to get her hanadamas and just something under $100 that she can wear daily, go to pearlparadise.com and look at the selection of freshwater pearl necklaces. You can find better bargains online than anywhere else, 99.99% of the time. If you want to see in person, go to a department store and browse, but buy online! When I started getting into pearls I liked akoyas better just because they were more valuable and I thought that meant something... but the more I started learning about pearls, I realized freshwaters are really your better bet.
Also, as far as necklace length goes... I am 5'1" and my akoyas are 17.5 inches. They're not overwhelming at all, but they are a smaller, graduated necklace (3.5-7mm). Not graduated, or for a more casual look, I would definitely go for 16". Along the same lines, keep in mind that a smaller mm will probably look better. 6 or 7mm will probably look best.