I've been selling and growing with Pro-Mix for over 30 years........Sunshine mix is the west coast product, Premier Pro-Mix is east coast.
Retail I pay around $35-$39 for a 3.8 cu. ft. bale.........this fills around 11-12 4 gallon pots. It comes with a fertilizer charge that should last around 10 days or so for seed starting. I always add additional lime, around 2+ cups of dolomite lime and additional perlite. I would highly recommend it.
The Pro-mix is fairly flexible, but since the pH tends to rise in the soil over time, it's best to stay low, around 6.2-6.3 pH with anything that goes into the pots. I've gone up to 6.5 pH with no issues- I would not go above that number. I let the very tops of the soil dry before watering and it's probably best NOT to let the soil dry completely out. The extra perlite helps the water drain well and stops the soil from compacting, which it will normally do over time. I also add Cal/Mag to every watering, between 5ml -10 ml depending on the plants needs.
Bottom line, it's a very forgiving product which you can push hard and still receive good results. If you use crappy and or/cheap ferts, you need to flush often so they don't build up in the soil. I use Botanicare Pure Blend Pro, which is mostly organic, and I never see fertilizer burn, nor do i ever flush except with plain water the last 10 days or so.
Check out my grows, you'll see some amazing buds with it...........