Figured I'd throw my experience/advice out there, even though this thread is already kinda insane at 1200+ posts.
In 2013 I wandered into a Nissan dealer based on Nissan's corporate site offering a $250/mo lease deal on an SV. I'm more a technologist than an environmentalist, and I've wanted an electric car since the days of Beyond Tomorrow on the Discovery channel but it wasn't a "must buy" for me.
Nearly 6 hours of "haggling" later (this dealership was quite literally criminal - story for another day), I got my $250/mo for 3 years with $2500 down. That's $11,500 total paid into the car for the lease.
In the summer of 2016 my lease was coming due. NMAC initially offered me a $17K buyout (less than what was on my original lease agreement). I told them I wasn't interested. They came back a couple of times, and eventually settled at $11,192 (inc'l taxes, etc).
I couldn't pass that up, as it was a total of $22,692 for a fully-spec'd SV with sticker price around $35K. Plus it had under 20K miles on the clock.
The entire buyout transaction took about 15 minutes with no surprises or nonsense (I went to a different and less sleazy dealer).
My current thinking is that if NMAC was willing to come down that low last year, I have to imagine that by 2020 they'll be willing to go even lower. Considering the number of EVs and PHEVs on the market (plus the 2018+ models), I can't see demand being high for used "107 mile" LEAFs three years from now.
It's just speculation FWIW, but I'd be willing to wager that NMAC won't want 2017s back by the time late 2020 rolls around, and will bend over backwards to unload them.
At the very least if you're considering buying out your lease now, haggle with them extensively.