I'd say a whole bike for £450 is a cracking deal!
Much depends on the model, size, colour, etc. But I'd say between £200-£350 for a bare frame, dependant on condition.
Pace frames were a steady development over the years, so there's some differences between frames, but generally a gradual progression in the design.
I'm a fan of the later ones, as the F7s & F8s take Discs as standard (but also rim brakes) - so good if you want a more 'modern' bike, especially as the F8 will take up to 100mm forks.
Paces are known for cracking at the seattube/seatstay junction, so look for one with a proper size seatpost, not shimmed. Also, try and get one with an anti-chainsuck plate fitted.
BTW this isn't an advert for my Pace in the 98+ section 
But to give you an idea of price I'm hoping to get around £550 for a really nice condition Red F8 frame, RC36 forks, RC45 Stem, original Syncros post, and Chris King headset (and maybe a couple of other bits like bars and a BB
