Hi,
So how much is something worth?
1) "Establish" a base price range (~$400);
2) How common /uncommon/rare the item is? (which should be reflected in (1) above)
3) The condition? (which should also be reflected in (1) above!)
4) Available Funds
5) ....AND most importantly, how BADLY do you want it?
As I'm not that familiar with the "scarcity" of the rounds themselves, the fact that it seems "complete" in the original container, original condition and heaps of round markings, normally would command a higher price.
As a "rough" comparison, a 2cm WW2 German complete flak round is worth ~$120-$150 AUD here in Oz (=USD right now!) - probably more now thanks to the Australian Customs Import ban! Your potential rounds, I would assume would also be in the same price range individually at least!!!. Throw in the container and the other bits and $400 seems a quite reasonable price.
(Remember the $400 was paid for a similar item several years ago!)
Lastly I gather these are aircraft rounds? (stenciling on the box lid), and aircraft ordnance seems a little more harder to find.....
Good luck - I hope you are able to "include" them into your collection!
Cheers
Drew
BTW - Have you "googled" for similar items for sale? The internet has a wealth of knowledge out there!