It wouldn't be worth it to me. I buy a jug of detergent whenever a brand I like (including sometimes "store brand") is on sale for, maybe, $2.50. With grown kids I don't have all that much "clothes laundry" to do, and there's only so much "house-stuff" laundry I have to do regularly. Then there's the occasional "once-in-awhile" kind of laundry. Sometimes I indulge myself by buying my "ultimate" brand (but even then it's when it's on sale).
I think I probably spend between about $35/$40 and, maybe, $60/$70 a year for detergent (and that's with using more than the bottles recommend per load). I'd be better off worrying more about some of the other stuff I spend on over the course of a year because even when two of my kids were here for awhile, and they'd sometimes (not always) use my detergent, I didn't see a whole lot of difference in how often I had to buy detergent. Sometimes, too, there's coupons or "buy-one/get-one" (that type of thing).
Either because there's a little left over from previous bottles, or because I don't have as many "special laundry projects" in some months, I pretty much think a $2.50 bottle last me three to four weeks. The $35 estimate if figuring ten months at $2.50 and a couple of months in which I may buy an additional bottle (and if I do that then there'll be quite a bit left when I take advantage of the next sale). I'm not above tipping seemingly empty bottles upside down and getting every last drop, though.
I don't know how much cheaper it would be for me to make it, but a) I have my favorite scents/brands, and b) the effort, alone, wouldn't be worth it. (I could save a whole lot more a whole lot more effortlessly by doings things like not buying "out" coffee at $2.00 a cup as often I do. :) ).