The price for the German Ebay one is roughly comparable to the ONLY other place I know of that sells a solo seat cowl for the gsf400, which is Wild Hair Accessories (they have a US website, but I don't have the link to hand).
The seat cowls are (I think) produced by the same firm - Gimbel in Germany.
No-one as far as I know produces a single seat unit. However, as soon as I have a single seat cowl, I might get a local fairings producer to mould one up. That'll be a month or two yet, though.