Update: This was mentioned a few days ago at $530-ish. It has since dropped to $484.
American Airlines, British Airways, and United have round-trip flights from Dallas (DFW) to Istanbul (IST) for $484-$490 with a connection. Flights January to April. Departures on Monday, Tuesday, Wednesday, and Sunday.
A Carry-on bag and also a personal item is allowed for free on American. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.