YES, the market has been flooded with D800,D800e, and D810 owners who are desperate to get those 45 million pixel files out of a new D850; I bought a used D800 for $795 a while back, and mentioned to braineack that the store I bought at had some great deals on D800 bodies. and he bought one from the same store, cross-continent, and also got a VERY good price on a super low-click (sub 10k!) D800. I agree, the D800 is tempting at the new-era,low prices, and I have to say, I have been tremendously impressed with the crop-in capabilities of the D800 versus the 24-MP FX file cameras (D3x,D610) that I have been using since the 2012 time frame.
Right NOW, as in late 2017, we are in the midst of yet another of what I call "the next big thing" transitions, when there is a new "big thing", one that brings a LOT of used gear into the sell-off-to-raise-cash segment of the used equipment market; we had it with the D100, with the D300, with the D200, and with the D800, and now, with the D850, when something with wide appeal to the bragging rights crowd, meaning those who demand the highest-end equipment in a segment, or the most-current technology.
If possible, sure, a D610 swap-out would be great for you.
Anyway...15mm...wow. When I was growing up, a 17mm was considered to be ultra-wide (and it still is to me!).