If you sell a product for $100 excluding VAT and make a margin of $25 on it, to convert your margin into a purchase or production cost expressed as a percentage of the turnover, you just have to follow these steps:
Start by dividing the margin by the turnover to obtain the margin rate: $25 / $100 = 25%.
To convert the margin into a cost expressed as a percentage of sales, simply do 1 - margin rate: 1 - 25% = 75%.