I think the going price for 804 Nautilus seems to be around $1500-2000... not much less than new 2CEs!
I haven't listened to 804s (any variant) enough to pass judgment. I have however, always had a fondness for the 802s over the 801s because of their base Quality, and I think the 804 series are similar to the older 802 in that respect. I listened to Vandersteens 2Ci, B&W 801 & 802, and KEF 107 &107/2 very closely several years ago, and I would have been happy with any of them, but the 801s tended to over load the room with base, but the 802s did everything as well as the 801s, but with better base control. The Vandersteens were very good, but couldn't quite "deliver the goods" at the lowest frequencies... though, to be fair, they were much less expensive. I ended up buying the KEF 107/2s because they produced base that was as well controlled as the 802s, but went lower... without overloading the room. I suspect the 804s are similar to the later 801s (having improved tweeters & enclosures), but without the "over the top" base of their bigger brothers.
As always, it's really best to compare for yourself, but from my passed experiences with both brands, I think if I HAD to choose blind, I'd go for the 804s.
Good luck, and let us know what you end up with.
Cliff