Apart from a few places in Kazakhstan or Uzbekistan, all other nationalists require a visa for entering Turkmenistan. To obtain the visa all the foreign citizens must produce a letter issued by a travel agency licensed in Turkmenistan.
The prices of the visa vary from one embassy to other however; these are cheap in comparison to obtain a visa on arrival. You need to get your visa six weeks before entering to Turkmenistan.
Both Tourists and Transit visas can be obtained easily on arrival at the airport. A transit visa will take 2 to 4 weeks to proceed and mostly it is valid for 5 days.
No transit visa is extendable however if you fall ill badly it can be considerable. The penalty for overstaying is US$200. You can get a tourist visa which is a little costly. You can only obtain it through only travel agencies with a license from the Turkmen government. Through a tourist visa, you can spend 3 to 5 weeks in the country.