It's normally £20 an eighth.
Ranging between £65-80 a half, and £110-135 an Oz. This is normally.
Currently, and for the last 6 months or so, it's been £25 an eight for the good stuff usually I found anything for £20 was of not such good quality, e.g wet/damp or contaminated import one way or another (watch for the stuff thats just "shake" with tiny little yellow bits in it. It's very common at the moment, and IS contaminated.)
Recently I've been paying £20 an eighth all the way up, even on the half and full Oz. Although it's been fucking DRO CHRONIC DANK.