Using a large cookie sheet and spread a box of baking soda on it. Put it in the oven at 400 degrees F for 1 hour. Let it cool and you have your own washing soda for much cheaper.
Hi - I am not really sure about how much saving after using my oven at 400 for an hour. Of course I usually just use a small amount of "soap" and add baking soda out of the box. Works great and not very expensive.
Here is an explanation of why this works and what you are actually doing.
Why do you have to put it in the oven first?
It changes the baking soda (sodium bicarbonate) into washing soda (sodium carbonate). Heating it breaks the bonds and removes water (steam) and carbon dioxide.