A profile pretty close to the Spurr GH mix is actually 1-part CNS17 hydro grow (NPK = 3-2-4), which can be had as cheap as $60 for 5 gallons.
Spurr is:
N 140
P 52
K 182
Ca 127
Mg 67
S 38
while
Botanicare CNS17 Hydro Grow (3-2-4)
@15mL/Gal
N 136.5
P 39
K 151.5
Ca 127.5
Mg 22.5
S 49.5
Seems just a little low on Mg and K. Add Mg with epsom at about 2 gram/gallon. Addition of 2-3 mil /gallon
Pro-tekt silica (0-0-3) boosts K up and has it's own benefit, but will raise pH-- so the addition of phosphoric pH down will contribute some ppm of P, and things will match very well, although the addition of epsom will imbalance the S from the Spurr target. Is there another cheap source of Mg that won't alter other macro nutes?? Mag oxide ?
Total cost here is gonna be real cheap.
Protekt can be had at ~$150 / 5 gallon, so total cost per gallon (using 15 ml cns17 hydro grow, 2 ml
Pro-tekt) is like ~6.5 cents per gallon (if bought in 5 gallon size) + cost of epsom (lifetime supply can be had for ~$20) and pH down..
100 gallon rez changed weekly for 10 weeks for less than $70, which is about the cost of .. use your imagination:icon_animal26:. Not as cheap as mixing everything on your own from scratch, but pretty good for a simple mix.
Results that desertsquirrel shows in the linked thread speak for themselves...