Promix is really nothing more than the base mix I outlined above, peat moss, perlite, and some lime. I have no experience with it (or any bagged mixes), because it didn't exist when I first learned to make my own mix 45+ years ago. The company that makes it did (Premier), and I have used their peat moss over the years, or, Lambert peat moss. Lowes or HD will have either. A 3.8cf bale is ~$13 at either place.
A bit harder to find at first is the 4cf bags of perlite. I'm in a small town of ~20k and there are still 2 places that carry it, a Feed & Seed and a builders supply. Perlite is also used as insulation, but is exactly the same as the horticultural grade. The 'chunky' is actually less effective than the regular stuff. Price is ~$18/4cf bag. The local hydro store gets it from the feed&seed and doubles the price to $36. LOL
Dolomite lime is ~$4.50/40lb bag at HD or Lowes.
Mix it yourself and it is well under 1/2 the cost of the promix, plus being a better mix (adequate amounts of both perlite and lime).