$592 is a really low transatlantic fare given the current fuel cost environment, let alone all the way to IST. I'd jump on that. Places like CDG are much more expensive around the same time.

For comparison pricing, I would look at carriers that actually operate to IST then at those that codeshare, rather than the other way around. Codeshares can be cheaper, but I've found it more likely that the web site of the operating carrier is likely to yield lower fares.