It all depends on how much messing about you want to do.
The Havas bus ,which is timed to meet domestic flights, does not go to the otogar, it only goes as far as the University.
You could catch the Havas to the city centre, ask for Konyaalti stop and take a taxi or you could go as far as the University and then catch a taxi from there.
There are taxi ranks near the stops.
Airport to Konyaalti I think is about 10 lira each and the taxi to the otogar is about 20lira.
From the University I would think the taxi would be about 10 lira.
A taxi from tha airport to the otogar will probably cost around 50 lira nowadays. I have not used one for a few years.