There can be a fairly large price range between an excellent cut diamond and an okay cut diamond. Unfortunately without more info, there isn't a whole lot that anyone can tell you on this stone. I have seen excellent cut 1 carat rounds go for around $6-10k, while mediocre cut 1.5 carat rounds can go for $7-9k or similar. Mall jewelers will sometimes try to sell you a mediocre cut 1.5c stone for $14-18k! If you are savvy you can get an excellent cut round online for around $5-6k. I don't know too much about Princess but I think the general idea is the same. Once you know more about the dimensions, table and depth, etc on this stone..I am sure that people can tell you more.
Good luck!