Far beyond the "buy on sale using a coupon" advice, there are several ways to save money on laundry detergent. One is to simply use less - way less. If you've cut back on using full scoops, that's the first step, but try this: Take a small item fresh from the washing machine and put it in a bowl of clear water. Swish it around a few times. Is the water still clear? If not, the chances are that you're using too much detergent. That excess detergent traps dirt and odors and can irritant your skin as well as cost you real money.
What to do? It's not cost effective, but you can do a double rinse each time, or you can cut back on detergent, or both. Being frugal, I prefer to cut back on detergent even more instead of wasting water rinsing out something that apparently isn't necessary to have in there.
Another solution (or an additional step) is to do a load now and then with no detergent at all. If your laundry is like most, there is enough detergent to do the job in the laundry itself plus some trapped in the washing machine. Try a load and you will see. It may seem all kinds of wrong to do that (I won't tell anyone if you do), but you might be amazed that your clothes seem to be just as clean as if you'd loaded the machine with detergent.
Of course, you can't wsah every time without detergent, but a load now and then won't hurt at all.
Another thing you can do, especially for dark clothes, is to wash them with a squirt of cheap dish washing liquid. A small squirt, mind you (about a tablespoon's worth), you don't want the laundry room flooded with suds. Dish washing detergent leaves fabrics soft and it cleans oils and surface dirt well.
I have made laundry detergent from the few "recipes" you can find online and I've used very generic detergent with more filler than detergent but using much less detergent to begin with and simply washing a load without detergent at all every third or fourth time, makes saving on detergent simple and easy.