I doubt if you will get a new 17" flat screen for less than �150 but you could try somewhere like e-bay. I have seen regular 17" monitors for as little as �35. Have you given any thought to using a 15" monitor? The price difference is considerable.
If you do buy a flat screen monitor try to get one with a glass front not plastic. They are easier to clean and I think they give better definition.
When PC Pro done a review of 17" TFT's, they reviewed the Relisys TL795 as their cheapest and best value, at �163 inc VAT. Type that make/model into Google and it'll find a supplier that does it for �150.
works great with half life 2, and general windows usage. only negative is the viewing angle, but you cant really get much better for this kind of price range.