It's not at all easy to do such a conversion. The 8 series' roof is a supporting element and removal will result in a weakened body. The car will start bending and squeaking and lose overall- and driving stability.
As far as I know there is only one company that does it properly:
Koenig in Munich. They install body reinforcements so that the car can take the additional stress caused by the missing roof. It does cost, of course. They charge EUR 85000, that's over $80000 (plus VAT of 16% in Germany).
The important question when you see such a conversion therefore is: "Who did it?"
...Mike