I would say £400 for the pair is a bit steep, you can pick up facelift for that price, I sold my prefacelift xenon's for £250 so price wise I would say £250 - £350
I was specific (but should be clearer) in my suggestion on the price if no hurry.
I'd expect to realise £400 by holding on for people needing a replacement. Doubt anyone will pay so much to upgrade due to other work needed, desire for clears and cheap aftermarket halogens.