I value it when developed early more than anything other than the chancellor and queen, so I suppose 7-8, but I recognise its value will decrease as pawns are exchanged, while the rook (the next best piece) will increase in value, so I expect in endgame the rook to be better.
The hawk is an interesting piece.
@CaptainTugwash is leaning towards 7/8 or even higher. I feel that it is only 5/6, but can still be a useful piece.