As of early 2015, a Turkish visa costs $20 in cash, according to the U.S. Embassy in Ankara. Anyone with a U.S. passport who travels to Turkey must obtain a visa at the port of entry. The visas are valid for any 90 days within a 180 day period.
Visitors who want to remain in Turkey longer than three months during a six month period are required to obtain a Turkish residence permit, the U.S. Embassy in Ankara explains. People already in the country should confer with the local Foreigners Police. Turkey strictly enforces its laws concerning foreigners. Penalties include fines, deportation and bans on future travel to the country.