If you want a new one, you'll be looking at paying out £841 at a reputable UK retailer. This Knoll Harry Bertoia Diamond Chair is currently much, much cheaper.
Obviously, this is a secondhand midcentury design, the original dating back no 1950. But according to the seller, this is an official Knoll Diamond Chair, which is the important bit.
Condition is described as 'good but not perfect' condition. with some wires at the front looking repaired and touched up (pictures on the main listing). There are also some age scratches and scruffs. So not as good as getting a new one, but at the present price of £6.29, an absolute bargain.