I E-mailed cars4youcustom.com and they told me 900 euros + VAT and shipping.
I asked how it was mounted and they said with "glue". From pics it also looks like it doesn't mount with the ST spoiler (they didn't answer that question) so you might need the regular fiesta spoiler. Also, it blocks most of your view out the rear, all of which leads me to think this isn't a great option for a street car.
However, Siebon is making full carbon replacement ST spoilers. They look pretty badass (not as badass as the WRC spoiler) but are only $600 (Euros?) or so. I kinda want to pick one of those up.