The original retail on that bike was $2000. It could be had at end of year sales for around $1700 to $1800. So there's a starting point.

Also to consider, Cannondale has replaced Caad 8 with Caad 9.

Assuming it is in great shape - it should have been specked with Ultegra 10 speed, Mavic Kysrium Elites and a Truvativ Roluer crankset and I beleive FSA aluminum bars (I'm going off memory) - I think $1000 to $1200 is probably a fair price.