How much is a 40 plus carat uncut black diamond worth?

Any diamond is worth what someone will pay you for it.

A black diamond is graded according to the classic four Cs of diamonds: cut, colour, clarity and carat weight.

A raw diamond looses about 60% of its carat weight during the cutting process.

You could find a diamond cutter willing to work with you to price and otherwise deal with your raw diamond.