OK, so I'll be the 1st to try and answer the OP's question. Unfortunately about £350 at most I would think.
It's not legitimate enough or eccentric enough to be valuable as a pre-Trek Klein (MC2, press fit bb etc), and not modern enough to be desirable (no disc mounts).
Despite the fancy paint job, it's not worth any more than any other 10 year old aluminium framed bike. Underneath, excluding the slightly fatter headtube (which just takes a standard headset) and square chainstays, it's not much different from a Trek or Fisher of the same era.
If you don't need to sell it, I would keep it. Sorry it's probably not the answer you were looking for. A shame as it's a decent enough bike, but anybody looking for a 'real' Klein would pass it by, and that's where the real money is.