I would have said "qu'est-ce que j'en sais ?" which is a little more 'casual' than "mais qu'est-ce que je sais ?"
(the meaning in that case is something like 'I don't know nothing about this, so stop asking me')
"Que sais-je ?" is the name of a famous reference book collection in France... could be interesting ( http://www.puf.com
It is a more litteral translation, grammatically correct, and it means a real questionning.
and what I was always told at school (in France, being french myself) is that the casual way is "est-ce que vous voulez..." and the formal, frenchly correct is "Voulez-vous..."