Depends how quickly you want to sell it. You could probably eventually find someone who would pay £100 if it's in immaculate condition. But advertise it at £50 then accept offers for a quick sale.
It is a shame that bikes depreciate so much. Doesn't help that you can buy a new bike for under £100. It'll be rubbish but it'll be new, and buyers like "new".
Whereabouts are you and what size is the frame?
~PB
21" frame (I think, I'm 5' 8" and it's a slight stretch). I'm in the west of Scotland