Have just come back from a scout round the charity shops in Alderley Edge, now I accept that there might be a bit of a mark up, a lot of the stuff is designer after all (that's why I went!) but in one particular charity shop chain (mentioning no names) the stuff was astronomical £90 and £139 even £18 for something from M&S!!
If you went to Selfridges towards the tail end of the sales you could probably get this designer stuff not second hand for a similar price (the £139 was a DVF dress).
I don't really expect to pay much more than £20 in a charity shop, or am I missing the point?