I started this about 8 months ago, excited for a cheaper solution that also meant I didn't have to lug heavy things from the grocery store on a regular basis. The added benefit of reducing the amount of detergents we use, saving plastic, ... all good too!!
I just did the math today to figure out how cheap it is.
I bought a natural, pure, olive oil soap. I spent 222kč (120kč + shipping; I actually spent less b/c I split shipping with my sister, but I'm rounding up) on 600g. I grate the soap and dissolve 12T of it into water to make a batch of soap. This is 35g, so I can make 17 jugs of laundry from one block.
The dissolved soap gets poured into a 1.5L container, and filled with water. I use 150mL of liquid (slightly more than a 1/2 cup) per load, so I get 10 loads of laundry from a jug.
This boils down to the price of 1.3kč per load. (This is a touch less than 7 US Cents a load, in my front loading washer.)
And Now, The Comparison
If you buy the cheapest Tesco laundry powder, it comes to 2kč a load, so this is pretty comparable But if you do what we did, and bought the expensive, pretty-smelling nice liquid detergent, it costs between 7-10kč per load.
If you do about 4 loads a week like us, this is 208 loads per year, costing 270kč ($14) using our homemade laundry soap. If you go for the nice stuff at, let's just say 8kč/load, the year's laundry will cost you 1664kč. What shall we do with our extra 1394kč? Hmmm....=)
By the way, this even beats the method linked here: a years worth of laundry detergent for $30, but of course I'm not sure exactly how many loads they are counting.
It seems like this third method with borax, washing soda, and dawn might even be cheaper than mine, at about $0.17 a gallon, which makes 32 loads. That's US prices so I am not sure how it would compare here, though I did just learn you can buy borax here in Prague. But I really like how mine is only 1 ingredient. Easy.