I am making laundry detergent today. I've been making my own laundry detergent for a couple of years now. This is the fourth batch I've made since moving into my current apartment - one five-gallon batch lasts me about six months. I know it's just me living here, but over the summer Isaac gets a lot of baths so we got through a fair amount of towels, and when Cayenne was living she went through a lot of towels due to her incontinence. It costs me about $2 to make six month's worth of detergent! I can't imagine why anyone buys laundry detergent when it is this cheap and easy to make.
If you've never made your own and would like to give it a try but aren't sure you want to go to the trouble of tracking down the supplies and don't want to end up with extra supplies you might never use, I sell a kit for making your own laundry detergent in my Etsy store. It costs a bit more for the kit than if you buy all the separate things yourself, but buying the kit is a good way to try it out once without having to buy a whole box of washing soda, for instance, when you only need one cup of it, and so forth. You can check it out here if you're interested.