Many factors.
1. The shape of the pearls: the nearer they are to being perfectly round, the more valuable
2. The similarity to each other: if the pearls in your strand look identical to each other in shape, size, color and shine (luster), it will greatly increase the necklace's value. If they are mixed or varied, it is less valuable
3. The shine: pearls with high luster look almost metallic- these are the most valuable.
4. The size of the pearls: the bigger they are, the more valuable they are.
And the list goes on. Pearls are complicated. Check the link below for more info.
http://www.shopgemstones.com/76.html