The Turkish Ministry of Culture actively promotes the annual Müzekart card, which allows you to visit more than 350 state museums and ancient cities in the country for only 300 lira ($6).
As Hürite mentions, this benefit only works for Turkish citizens and those with residence permits, while for foreign tourists without residency, the entrance to just one museum (for example, Ephesus or Topkapi Palace) costs tens of euros.