There are many developing urban areas and building stocks in the surrounding districts of Istanbul. However, residing within the provincial borders of Istanbul does not really give the feeling that you are living in Istanbul. I live in literally the center of Istanbul. If you do not want to spend 2-4 hours a day on the road and you have a familyr, unfortunately, it is impossible to find good housing for less than $ 250,000.
This figures increases exponentially if your interest is about residing in iconic districts. Moreover, this is despite the post-Kovid period and dollar develuation. For example, even now, if you want to buy a 4-room house from Dragos district -which is 30 km from Istanbul strait but it is a good neighborhood in its area- you have to sacrifice around 1 million dollars. If you want Istanbul strait sea side, the numbers are incredibly exorbitant. The mansion line solding around tens of millions of dollars, while the houses on the hills and ridges cost millions...If you say I just need to be close to the subway line, and say it is not important that the district where I live is decent, the prices go down to normal levels.
IMO, if you do not think for long-term investment, property prices in Istanbul are unnecessarily expensive. Izmir and Antalya offer better opportunities in this respect.