For a visitor with a limited finance, here is the options:

Cankaya Lokantasi: It is situated near the Atakule, and you can find it easily. It is really famous for Turkish cuisine.It is managed by six guys from the same family. Their father used to be a cook for the second president of Turkey "Ismet Inonu". The prices are around 13-14 liras for a meat meal with rice. You have to try the vegetable meals such as cabbage filled with rice(kabak dolma), mixed vegetables (Turlu), doner kebab. It is only open between 11.00 and 16.00. The service is very quick but you can wait if you want to sit down because of the limited number of tables. The takeaway is an option. The place is very good for vegetarians.

Ozge Piknik: It is situated in Kizilay on Gazi Mustafa Kemal (GMK) Boulevard. There is not enough place for sitting. But its doner kebab (meat) is really nice. It is open betweeen 10.00 and 20.00. The price of a doner kebap with bread is 7.00 liras.

Aba Piknik: The term "piknik" is very common for small diners around Ankara who sells doner kebab and sandwiches. But these are not in the same quality and price. Aba Piknik is one of largest chain restaurants in Ankara. They are selling a wide-range of sandwiches and also pastries such as lahmacun and pide.They have a special farming branch and they use their own cattles for the meats.The prices are around 10 TL with drinks.You can find it in large malls and city center (Kizilay).

 Hosta Piknik: Hosta is one of the oldest chain fast food restaurants in Ankara.Like Aba Piknik they have a large menu for fast foods. Their doner kebab is famous for not using minced meat in preparation. You have to pay a sum of 10 liras for a menu with drink (non-alchololic).

 The cheapest option is the simit cafes named after a traditional turkish pastry with sesame named as "simit". You can find them in almost all parts of the city. You can order one or two simit with a creamy triangle cheese named as "karper" and a cup of tea just for 2 liras. It is a tasty and cheap selection if simits are hot enough. There is also simit corners around the city operated by municipality but you can't expect hot simits from these places.