Not sure about Google Flights, but I often find that domestic TK flights are mispriced on Kayak/ITA Matrix. For example, I recently had to fly IST-ADA. On the TK website, there were plenty of economy tickets for around 30 EUR and business class for 70 EUR. On Kayak/ITA, listed prices were approximately 3 times higher. I would always check the TK website for such flights to be sure it isn't some kind of fare mismatch with OTAs/ITA only showing full flex fares or so.