The price per carat greatly depends on the quality, shape and weight of the rough. Determining rough value was very difficult for me personally. You need to know the wholesale value of the finished stone and what size and shape you can finish the stone in to determine value of the rough. Each piece really is, to an extent, unique in that regard.
If you want valuable finished stones expect to pay a premium for your rough. A turd is still a turd even after it’s polished. I’m talking $100 plus per ct for good, rare rough. Clean, vivid intense green emeralds for example. I’m actually in the market right now for a stone I’m willing to pay $500 per ct for the rough. It’s proving difficult though so I may be buying it finished from a European collector. But that’s another story 🙂