I'd hate to say this, but I will for the benefit of someone who will read this. Tracking flights and sales over some time period, waiting to buy, and not jumping the gun really pays off. Turkish Air has several sales throughout the year. It's worth it to be patient and not to strike too early, especially for off-season travel. I've flown Turkish Air twice to Istanbul and paid $524 for June and $763 (yes, with all taxes included) for October travel. I see the sales come up seasonally and, when they do, the flights to Istanbul are rarely above $850 for an economy ticket whether it's Istanbul or another European city. Often they are much lower than that. Caveat: this is from the Washington DC area, although they have sales from just about every market they serve (Houston, Atlanta, Los Angeles, etc). Turkish Air doesn't advertise their fall sales this early in the year (no airline that I know of does either).
October is not considered high season, and Turkish Air has prices that should beat any legacy airline by a few hundred dollars. The only time I paid $1,100 to get to Istanbul was when I didn't know Turkish Air existed and I flew Lufthansa (and booked the ticket literally one week out for summer travel - it was a spontaneous trip).