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Oxyclean is just sodium percarbonate with filler added. You can buy sodium percarbonate directly from amazon that is much more concentrated and cost efficient.
Magic Eraser-They're just melamine sponges. You can order hundreds for like 8 dollars. Magic Erasers has some kind of cleaning product in them since it creates a sort of "foam" when you wet it and start scrubbing. It's actually when wet, the melamine will start to break down. The fine substance yields a slight abrasive content.
The cheaper ones are usually a lot less dense. That means they wear out quickly. They work just as well though, and it's still a much better deal in terms of amount cleaned per dollar.
Here are a few links if you would like to get some