There is no fixed bus ticket fare system in Turkey. Bus ticket fares are depending on the distance, bus company, time and day.
For example I will list 3 different routes with 3 different bus companies:
Istanbul to Ankara (454km – 7 hours)
KamilKoc: 47-52 TL (19-22 Euro)
Metro: 40-45 TL (16-19 Euro)
Varan: 52 TL (22 Euro)
Istanbul to Izmir (564km – 9 hours)
KamilKoc: 55 TL (23 Euro)
Metro: 60 TL (25 Euro)
Varan: 55 TL (23 Euro)
Istanbul to Antalya (724km – 12 hours)
KamilKoc: 70TL (30 Euro)
Metro: 70 TL (30 Euro)
Varan: 65 TL (27 Euro)
So as you see we approximately say every 100 kilometers costs 10TL which is 4,5 Euro (€).
Ulusoy and Metro are leading bus companies of Turkey. You can check their pages for latest ticket prices and times. Also you can buy your ticket online via their web pages. And, yes they have English web pages.
(* Prices might change in the future depending on gas prices. 1 EUR = 2.36 TL)
To see all information about buses in Turkey and Istanbul, click here!