Travel by comfortable, modern bus in Turkey costs about TL8 per 100 km (62 miles).
Inter-city bus fares vary by company, by route and even, in some cases, by time of day, or day of the week, of by the equipment used (more luxurious buses may cost a bit more).
Discounts are sometimes offered for round-trip/return travel, children, special promotions, and for tickets purchased from the bus company’s website.
Here’s how to buy bus tickets in Turkey.
Here are routes, fares and travel times from Istanbul to other cities. Here are the major bus companies, some of which have websites with schedules and fares. (Sorry, I don’t have any more information than this.)