8th - 3.5.g (well should be 3.5) should be 20 - 25 GBP
Well that's the cost for Green anyway, in my area at least.
but....
if you're buying solids, the cost is usually quite a bit less.
8th of soapbar for example is something like 10 GBP
not entirely sure as i stay as far away as possible from that kind of crap.