Thank you for submitting your item for appraisal. This appears to be a Yongle style sweet white glaze concealed dragon pair stem cup. The hidden blue white painting of two dragons and a flaming pearl concealed under the opaque glaze would be typical for the Yongle period. The Nei Fu 内府 mark suggests that this cup was used in the imperial household. What I would find odd is the shape / outline of the cup, this could be a distortion of the camera lens or rightfully a doubt concerning the age of this cup. If it was an authentic piece of the Ming dynasty, Yongle reign (1403-1424) it would be very expensive and you should definitely show it to a major auction house in your area. My estimate would be a very conservative one for a well done 20th century reproduction.