Like anything in this world, you get what you pay for, you should be able to pick up something second hand for around ?100, it might be old but it will usually have a metal body and glass lenses, more modern scopes are plastic with plastic lenses, unless you are spending a fortune.
Greencat, Opticron, Hawk all have lots of models between them. if you want maximum sharpness, go for fixed FL rather than zoom, 20x to 25x magnification is all you need to see the nocks at 100yds. 50 to 80mm objective lens (front element), the bigger the better as the bigger lenses will let in more light. I get lots of Oohs and Aars when people look through my scope (a Hawk something or other) but my previous scope (opticron I think, an old 'un but good 'un) was much better and a bit cheaper. Once you have looked through a decent scope you will realise why there is a price difference. As has been said already, if you really want to have a bad day, try a leica or Swarovsky, then spend the rest of your life dreaming.