For honest salesmanship and good-quality local textiles, family-run Atelier El Bahia is a great choice. It's a proper emporium with fixed prices, made-to-measure service and helpful English-speaking staff: look for the weavers working hand looms in the front window and don't be shy to come in and ask questions.
Second-rate textiles cost around Dh250 per metre, while luscious cottons and silks cost Dh600 to Dh800.