I have got a feeling that the only airline that flies to Northern Turkey is Turkish Airlines. This is because the country technically doesn't exist, and other airlines are prevented by United Nation's sanctions from flying there. This is all to do with the Turkish invasion of Cyprus back in the 70's.

Edit> I've found some more information :-
Removed for copyright reasons, but the info may be found by clicking HERE.
David HT Mod

So you will not find any cheaper flights !

Thanks, but I have heard this evening that Easy jet are starting flights to Istanbul with a view to working with another airline to provide a cheaper alternative, so fingers crossed.