I'm not familiar with the R6400. This is the first I've heard of it. It looks to have the features of a R6500.
I'm going to guess that it has a Government profile barrel, and a AR15-A2 marked lower. Probably has a "SP3XX,XXX" serial number.
If so, I'd guess that it's a R6500 that has a misprinted label.
At that, R6500s are about the lest desireable green labels. I'd guess a NIB version would retail for $1400, maybe $1500.
If it is some rare version, that is somehow different than a R6500 (other than the label on the box), I guess it would command some sort of premium. Not much, as 20" guns with A1 sights are not in demand (as there are about 300,000 SP1s that look almost just like it).
I would like to think that I'm a collector, even if my collection is a bit anemic. One of the guns that is not in my collection is a R6500. I'd probably pay about $1200 for one, NIB. I'd probably only pay a $50 premium for a mismarked label. Now, if your gun (not box, but gun) was marked weird, or if it was something other than a R6500, I'd be more interested in opening up the wallet wider. If it was 222 instead of 223, I could come up with about 50% more money. Hope this helps a bit.