Determining worth is a difficult thing with the BT. How badly do you want?
If you try your luck with Japanese auctions, they can range from CAD $18,000 to CAD $25,000. Expect to put in some money for perishables and renewables. How exacting a driver are you? I am and can tell you how much I have put into my car since buying it about 2 years ago.
For a BT which has had regular servicing performed (check for a thick stack of receipts) and many mileage issues addressed, I don't think you should expect to pay less than CAD $35,000.
This puts it significantly more than other great condition E34 M5s out there. About twice as expensive. Back to question #1: how badly do you want a BiTurbo?
On the other hand, once a BT is sorted out, I strongly feel it costs less to operate than an equivalent E34 M5. That's my opinion only. The E34 M5 is much more exciting aurally than a BT but not as fast and stock for stock, will never be as fast as a BT. If you drive long distances regularly, the BT is a superior car. If you track, the M5 is better and more rewarding. The BT is more quiet and refined. The BT is also harder to re-sell.