Check out your country's labor laws. In my country (Malaysia), working overtime is 1.5x wages, working 7 days a week is 2x wages, and working on a public holiday is 3x wagesIn my country (Malaysia), working overtime is 1.5x wages, working 7 days a week is 2x wages, and working on a public holiday is 3x wages.
This is the absolute minimum legal price increase for a developing country. If you live in a developed country with better labor protection, charge more. In Europe, double price for working your weekends off is probably cheap.