The thing that drives how much you will pay, will be what you use your mobile for? If it is just to make calls and no surfing/emails then you can pick up some cheap chinese/turkish handsets here for somewhere north of a couple of hundred. If you want it for calls and surfing then you are looking around 500-600, probably include a camera too. If you want a good quality android for calls, surfing, and for all your favourite apps, plus a good camera then there are some fine non mega brands handsets available for around 1200 lira.
Places to look Kipa, the 'electronics' shop in the shopping mall, or even online if you can via sites like Hepsiburada, ALSO keep an eye out in Bims or A101 who often have some good deals on mobile handsets, and if you haven't got one already BIMS has the cheapest sim and calls/data packages in Turkey.
I have brought in a UK phone and went through all the hassle of registering, taxing and advising Turkcell. That will cost you around 200TL these days, and that will go a long way towards a new handset, with a lot less hassle chasing around town. If you do decide to register UK a post here on CBF how to undertake the necessary steps on CBF, but it was a few years back.