I follow CS & CSS prices closely, price wise, they seem very spec dependent, other than the usual factors of mileage, condition, fsh, etc which affect all GTi's.
Now they are around 3 years old, having come off of lease deals, I have seen a couple of £20k cars with close to 50k miles and basic spec.
I was keen on a CSS, but didn't need a third 2 seater, so bought a CS in the closest spec to a CSS as I could find.