Depends. Cheapest is around £200 for an compact Acer one, but from £300-500 is what you can expect on average to pay. Obviously that's not including macbooks and shit.
What is your usage? If it is just for using the web and documents then I would go for one of those smaller machines with the SSD, you will notice how zippy they are instantly.
It's a nice cheap replacement that should serve you well. You can get some pretty decent machines for £300 or too, but again you will be limited in what you can use them for.
I'm a MacBook Pro user, I don't think that I could move away from this now, maybe to a *nix OS, but never to Windows.