How much should a projector screen cost compared to the projector?

Hi all,

Later this month I'm about to buy my first projector and a projector screen.

So far my battle for projector is either:

1) The Hitachi pj-tx200 or Sanyo Z4 (cost about 1,300£ in Denmark)

or if I feel like spending a bit more it would be the

2) Sony hs60 (cost about 2,100£ in Denmark)

Now... These are all 16:9 projectors, so Im thinking about buying a quality screen 16:9 projector screen (white with black around the screen) for about 500£ - is that enough to get the right quality from the projectors or do I have to aim higher?

But how much should one normally spend on the projector screen compared to the price of the projector?