Personally, I wouldn't worry about coming up with a price until I had the upper in my hands. Given Ares incredibly poor track record of following through with delivering product and the fact that you are dealing with a friend, I'd wait until I actually had the item before making a deal. He knows that you're paying $2,500. Depending on your level of friendship, is he good with you over doubling your money with him? He might not say it, but is he looking to use his relationship with you for a "super buddy" price? It would be a drag to put stress on your friendship, and then have Ares fold and stiff you. So your friend thinks you should have offered him a better deal, which he takes personally and then Ares doesn't deliver your upper. That is the worst case scenario, but why risk that? If all these uppers actually get delivered there will be plenty going up for sale to give you an idea of market value.
I'm still not sure as I understand why you want to sell. I'd live with the Shrike for a while. The MM 23 looks to be a great 23E clone. The last I knew the MM 23 was selling at $12,000. You have got to shoot your buddy's MM 23 but you have not "lived with" a from the shoulder 5.56 belt fed gun. For me, I've found the borrowing and owning a firearm is two different things. And a shoulder fired belt fed 5.56 is a different kind of firearm. After "living with" the Shrike, will tell you whether a shoulder fired 5.56 is what you want in your collection long term. At $2,500 the Shrike sounds like a much more cost effective way to see if this style of firearm is what you want in your collection. If not, you can still sell the Shrike for a good profit and put the money into another direction. Far be it for me to tell another member how to build his collection, but it would cost you a lot less with the Shrike to find out if a shoulder fired 5.56 belt fed machinegun is for you, in the long term. Good luck with the sale price of your Shrike for your friend.
Scott