Back in the day, the rule of thumb in textile costing was "double, double, plus 10"(at least I think it was +10... I took that class some 15 years ago.) In other words, if it costs $5 in raw materials, labor, and manufacturing overhead to produce a garment, you double that ($10) then double that ($20) then add 10% and you have a full retail price of $22. This allows for a slim profit margin after you weigh in the costs of design, logistics, merchandising, unsold goods (some sizes don't sell by the end of the season and they end up at 80% off to get them out of the store), the retailer's stake, standard markdowns, etc.
There are of course more specific calculations used, but the rule of thumb gets you in the ballpark.