I wonder whether you have hints regarding budget accomodation on any of the Princes' Islands outside Istanbul. We foresee staying there for two nights in middle-March.

I hope this link helps you
http://www.turkeytravelplanner.com/phpBB2/viewtopic.php?f=8&t=2618

http://www.tripadvisor.com/Tourism-g635899-Princes_Islands-Vacations.html
and this.
might find some personal opinions on accomodations.
as a starting point will do good, i think, but i hope someone will pop up with better infos.
good luck.