This is really an apples to oranges debate I think. Stuff like
promix is cheaper but requires feeding pretty much immediately.
FFOF has enough food to grow for a month. That's a fair bit of nutrients saved and that adds up. That said
FFOF is way overpriced in Canada and on the internet, and I can see where you are coming from Pipecarver. I couldnt justify the price either and dont like the lack of control I'd have growing with it either. But the price gap is much smaller in the US and other places, and it has earned a reputation among pot growers.
Promix, or the mix Pipecarver has is for soiless hydroponic growing.
FFOF is a potting soil mix (albeit made with a soiless mix as a base) with organic amendments and added nutrients, designed to be used as a water only, sort of "supersoil" type of potting mix. They're two different types of media altogether.
I would class things in terms of; Potting soil mixes like FFHF or OF, or super soil etc.
Soiless hydroponics, growing media with a CEC and buffers, but no nutrient value like
Promix, proterra, Coco, etc..
Then substrate based inert hydroponics like rockwool, hydroton etc. Finally Straight water based like DWC or aero.
The lightbulb really went on for me lately when I realized $40 worth of coco and $16 worth of fabric pots will grow me the same crop as $150 worth of rockwool. That's a much closer comparison than
promix vs
FFOF.