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 wages][1].

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.


  [1]: http://www.ilo.org/dyn/natlex/docs/WEBTEXT/48055/66265/E55mys01.htm