Every day, there are direct bus services from Istanbul, Ankara, Izmir to Kusadasi at different times during the day. There are different bus companies that serve these routes. I would prefer Ulusoy or Varan. I used to prefer Pamukkale.
If you prefer to travel by bus, there is something to be aware of, there are a lot of breaks on the road, the passenger loading overload is very much annoying, after a while. In order not to be exposed to this situation, be sure to choose a non-stop service, please confirm this subject in advance. Bus ticket prices will go from Istanbul and Ankara to 200-250 TL round trip. For those departing from Izmir, round trip is between 40-50 TL.
Istanbul-Kusadasi: 550 km
Izmir-Kusadasi: 90 km