oil leak e46

Hi guys

need your help please. I have a damn oil leak somewhere and am unable to find it...i have replaced the top gasket already but it seems to be leaking at the back of motor somewhere...any advice on what it could possibly be as a preventative measure...or would you suggest i strip everything and do fault finding from there...

any idea how much a full gasket set would cost? i am thinking it could even be a brittle oil pipe...