For about the price of the Formulary kit, you can go to Costco and buy 1000 tablets, 500 mg each, of acetaminophen, then the local grocery store and get a pound of Red Devil Lye, then a pool supply shop and get a pound of sodium sulfite (chlorine reducer, but be careful because sodium thiosulfate is also sold for this purpose) -- or you can order in the sulfite from a number of places, including Formulary, though shipping might blow your $20 budget.
When you have those ingredients, though, and some distilled water, you'll have everything you need to make
Parodinal . Your pro-rated cost for the concentrate, which you'll use exactly like commercial Rodinal (but which only lasts about 90 days before it starts to weaken) will be about 30 cents for 100 ml, if like me you pay approximately $1 per gallon for the distilled water. And you'll have materials in hand to make several gallons of concentrate, though I'd suggest making it 100 ml at a time due to its limited life.
Of course, you won't be likely to use up the lye before you have to buy more -- it absorbs water and carbon dioxide from the air every time you open the container, and when it starts to clump badly you'll need to replace it because of the hazard of flying particles if you try to chip off bits -- but even if you figure that in, it will cost you less than a dollar for 100 ml of concentrate.