I think $100 for an A4091 is crazy. You could buy a whole A4000 for that on a good day. If price is no object, maybe.
The A2091 is an equivalent card for the A2000 and you can barely give these away (they are not useless, just not special - and they too have their quirks).
2091 cards are bring $30 to $50 now on ebay and ebid. Depends on how bad you want/need one I guess. 2091 can work in an A4000 but add a new meaning to slow.
I don't guess it make any difference if it's a Commodore or DKB card - I think they are the same. The last 4091 I saw sell went for $25.00 with a cable.
I just happen to have two cables (remember they are HD50 ports) 1 for 25 pin & 1 for 50 pin centronics.