Every product page shows the price per garment for each box size, VAT included. Compare it with the price you sell at: the difference, minus shipping and your channel's fees, is your margin. The bigger the box, the lower the price per garment.
In the new lots every garment keeps its tag with the shop price, so you know what it is worth before you buy. A common rule among resellers is to list at 50% of that price.
The profit calculator does the maths for you, and The margin on a 20-dress pack walks through a full example with several scenarios.