OSX and iLife account for £140 of difference. However, windows is not 'free' so you can take a figure of your choosing off that- perhaps £25. A new mac comes with an approx £115's worth of retail price product.
But - you can take £15 off, because to use an external monitor you'll need either a VGA or DVI adaptor - something I've never seen required on a PC laptop. £100 difference is justified based on the OS and iLife - perhaps 10-15% over a PC laptop. We can add a little more with the iSight (£25), Bluetooth (£15), Magsafe (£10)
More beautiful? The number of cracks that appear on Macbook cases - that's a dubious claim. I have one I've not seen mentioned on the top, front left, growing toward the keyboard. I'm none too beautiful when I have to bang the left arrow key to get it to work properly either. Perhaps we can say the value of the looks is the same as a nice polygfx skin - £30.
In terms of tangible, accountable different - I think £180 is a fair figure. Now - considering any PC laptop for more than £350 will include a DVD writer - the only Macbook we can compare with is the middle priced £829 machine. Less the £180 Apple benefits figure - £649. That sort of figure can buy you a good PC laptop - with a webcam, proper graphics acceleration, etc etc. A Macbook like specification will probably set you back £499 - £330 less than an actual Macbook.
I've tried to make the 'sell' to friends and family - shown them OSX, shown them iLife. But when you tell them that it means they'll be paying 40, 50%+ more for the machine - they ignore it. My office colleague just bought a PC laptop with MBP like spec for £700 less than the middle-spec MBP. At some point, you just have to say 'Because I'm prepared to be ripped off'. I am.
Doug