Alexander,
I only know from my own experience but unless you are willing to live quite far out, it will be difficult to find somewhere for under 300 euros.
Then again you could get lucky as a few of my friends have. I wouldn´t advise takin somewhere on until you´ve seen it- stayin a hostel for a few days- work out what areas you are prepared to live in. Get up early to buy segunda mano and just basically work your way through every number- see as much as you can and don´t panic yourself into taking something you don´t like or won´t be able to afford.
Lots of luck,
Aine