Okay, let me re-phrase it a bit: If you were offered £40 to work one day extra a week, would you?
It's all down to what you earn and what you need to earn that decides whether or not you'd charge high for a weekend work, I guess. Percentages, if you will. Why should I work for peanuts at the weekend if I don't need to? Why should anyone?
Also, everyone always thinks that all the materials they choose are top quality. They're usually wrong. I'm sure that's not the case with you of course, just a generalisation.