Frankfurt is still relatively cheap compared to London. But you are right, if you want cheap rent then Berlin is unbeatable. Also probably has a better startup scene than London at this point. (if that's what you are looking for)
I've lived in both Berlin and London, and while Berlin definitely has much cheaper rent and some great startups (Amen, Soundcloud, etc.) the London startup scene is still larger by an order of magnitude on pretty much any measure (number of startups, money raised, exits, etc.)
maybe some years ago Berlin was much cheaper, but rents are getting more expensive everyday. Still cheaper than London though.
how much is rent running these days for a 2 bedroom (3 ZKB)?