But beyond that, the debate regarding synthetics/semi-synthetics here has become ridiculous. Both have been used by designers for decades now, and during that period there have been significant technological advances in textile production. They weren't adopted solely because they were cheaper—they were adopted because designers saw that they had distinct qualities which could lead to new techniques in garment construction, or because they presented new colours and textures (etc). So there's a symbiosis or feedback loop whereby designers, in making use of these capacities, not only discover new techniques but push forward the development of still newer fabrics.
And the question of technique or construction—and hence labour—is the other missing element here. Yes, a synthetic/semi-synthetic will typically be cheaper than a silk or wool, but that doesn't take into account the factor of labour. If the use of viscose or polyester involves couture-level techniques, then *skilled labour* is the major factor or cost, and there's the very real possibility that a viscose garment will be more labour-intensive and hence more expensive.