If you want advice, you're going to have to provide more information. Crown and pavilion angles for starters (the certificate number ideally). Also, people will probably ask if you have an ideal scope image and picture of the diamond (or if you've at least seen it in person).
It's probably best to take it to an independent appraiser. They will hopefully have the equipment necessary to determine whether the diamond is well cut or not (idealscope, sarin, hearts and arrows viewer etc.).
Without knowing more about the specs of the diamond, there is no way to tell if that's a fair price.
Based on a quick search, I'd say it would be possible to buy an ideal cut 1.7 G SI1 for around $15k. So it may be a fair price if the diamond is well cut.