I flew to LTBH last Sep.
From some emails with locals, I have this
For Istanbul head for Sabiha Gokcen LTFJ
Handling & airport charges total varies 120~180 Euro if I remember well.
AVGAS is available but with prior arangement from the Aeroclub.
I know for sure that LTFJ has an avgas bowser - it was parked at
LTBH during the
2009 fly-in and they were flogging the juice at an interesting price
Google on
istanbul aviation club for contacts.
As always, and particularly in the 3rd World, never fly anywhere unless you have contacted the airport first.
Turkey is 24hrs PPR for all non-EU reg planes, IIRC. You get a numeric code which you read to ATC on your way in.