It will very likely cost around 24-25 euros now, but make sure you agree a price before getting in a taxi. They quote approx. 21 euros for Playa de las Americas.
http://www.spanish-airport-guide.com/ten…
Alternatively, you might consider the bus from the airport to Costa Adeje, which will only cost about 2 or 3 euros.
http://www.titsa.com/lineas.asp?idl=488

