Hi Mat,
last time I contacted Aeroflot by email with this question. They said "USD 100 per leg + fare difference".
Since you're in E, there's also no option to upgrade with miles if I'm well informed.
I personally don't appreciate Aeroflot's "Comfort" Premium Economy class for night flights. Day flights are more interesting because you get enough space if you want to work with a laptop.
I'd always prefer a Space+ seat in Eco or 18C/H over "Comfort" class.