You can buy a new CAAD 8 bike for £500 and a new CAAD 10 bike for £1,000 so a new CAAD 9 bike would probably be about £750 if you could find one. Assume half the cost of the bike would be the frame/forks, then the frame/forks would be about £375 if you could buy them separately. So I would say that a used frame/forks in very good condition would be something below that, say £250 - £300?
Really though, the value is what you or someone else is willing to pay. I bought a CAAD 5 frame/forks for £500 about 13 years ago which was a bit over the odds, but it was my size and I wanted to have a Cannondale built to my spec so I paid it.
PS I still love the bike, and I'm sure that a CAAD 8 would be better still, so if you are buying - go for it!
AFAIK Cannondale do not sell frames that are not pre-built into bikes so there is not an official new price but you shouldn't price your frame so high that it would be cheaper to buy a bike and sell the unwanted parts!