Isn't 400$ for just the case pretty expensive?
Yeah, it was $220 for the whole kit not counting shipping and fees. I would be all over this if the price wasn't so high, somebody will most likely pick it up.
What do you think would be a more fair price?
$200 shipped.

As someone mentioned earlier, the full kit was $220. The most
I would pay, just for the case, is $200.
And just for the record, I picked up a full Duck Viper kit, with stabs, a full cherry keyset, AND a clack for about the same price that you are trying to sell just the case for. I have seen multiple, fully assembled Vipers go for as much as you are asking, for just the case.
I don't mean to threadcrap, but my post is more or less a heads up to newer members that might not be aware how marked up your price is.
Granted, this case is on the rarer side of cases, that is still a hefty markup.
edit: I also picked up a mint Duck Mini v2 case for $160 from the classifieds.