Simon,
My set-up is A/V biased, so it's not fair to compare unless you're going for visual as well as audio.
Audio gets to a point of diminishing returns - you will spend double but he improvement in sound will not be twice as good, and in most cases you won't noitce a massive difference. The key is to price your system at the point where spending more will not gain you much...
If this will be your primary system and you love music and want to put a budget against a system that you will not want to replace in a few years, I think you should consider assigning the following rough costs: -
(keep in mind this is a VERY good system and it can be done for A LOT less, should you not want to spend semi-silly money.... equally, you can spend 3x the money)
CD Player (Single) - £1k
Amp £1.5k
Speakers (x2) £1k
Gastro, another good way to do this is to speak with some who buys in pre-owned equipment. Many good HiFi shops allow trading-up and thus totally A/V buffs will buy a £3K amp and up grage it when something 'better' comes along - you then pick up a brilliant piece of equipment, that will be like new, for half the price!