As if it wasn''t enough to come with giant faceted ruby and sapphires.... This one should be off your hands before any auction house gets a grip: there are so few cabs anywhere close. Sure you know that all too well. I can''t see any reason why this would be priced significantly different than a faceted sapphire of similar quality, counting flaws & all.
If of interest, there have been two pieces of signed jewelry built with large sapphire cabs chosen for advertising campaigns lately: one in a necklace made by Cartier to celebrate their latest anniversary, one in a similar show piece by Boucheron. You may be told that cabochons are not in fashion... Well, it seems that they are. Neither of those pieces even came close. You may be able to track down those prices for reference if interested.