I am quite a nitwit when it comes to prices.
I can only tell that I have never seen these projectiles in a museum mint condition like this. I can ashure you they are rare, as they were only made as small batches of experimental shells.
Over here I have only seen them in "polyester and sandpaper repair" condition, mostly with the upper or lower sabot missing. Even than, high prices like 300-500 euro are asked.
Also, even if they'd be available here, it would be quite impossible to have them shipped over to the US of A due to postage and customs problems, which makes them even more expensive on your side of the pond.
If I would have to make a carefull estimation, i'd say at least $750 or higher each in Europe, taking into account the often crazy prices asked -and paid- on USA auctions i'd say $2000 or higher each in the USA.
A piece of advice to your friend; do not sell them, he'll not ever find a nice set like this again in this condition.