500g+ of homemade powder
1 cup borax (sodium tetraborate decahydrate)
1.5 cup soda crystals (sodium carbonate)
1 cup grated soap
I use more of the soda crystals because it's cheaper than borax and softens hard water, and I make powder instead of the liquid detergent, because the liquid isn't strong enough with the hard water we have here.
One (very long) bar of pure soap can be bought for 99p in Asian shops. One bag of soda crystals cost 90p in Asda, I use about 1/4 of a bag (250g). Borax is the expensive ingredient: 1 cup is about 250g, but buying in bulk on eBay is the cheapest, where you can get a kilo for £6.25 including p&p.
So, for this recipe the value would be £1.5625 for the borax, 22.5p for the soda crystals and 1/30th of the soap bar; 3.3p = £1.8205, under £2 for half a kilo.
Now, how does that compare with store bought?
The cheapest in Asda: Asda smart price biological automatic washing powder costs 79p for 800g and the most expensive Bold 2in1 costs £2.26 per kilo (£1.13 for 500g).
Now that was disappointing :(
Conclusion: if I've done my Maths (& measuring) properly, it's not at all cheaper to do it yourself, but then again there are a few more factors to add to the equation: efficacy of the different soaps, environmentally friendly alternatives (?), knowing what's in the soap, muscle-toning whilst grating the soap(!), and you could save momentarily if you use up what you've got at home instead of buying new.
I might try less borax next and keep track on how many washes i can get out of this.