Fox Farm is bad ass shit!! It has treated me right for a long time now. I did a bit of a study between using just the original potting soil, and just the ocean forest (no perlite or anything added) side by side on the same seeds to see what worked best,and the ocean forest definately had the advantage in growth rate, however I could tell NO difference between the 2 when puffing on it, I've never added peat moss to it, so I cant tell you weather or not to use it, but roughly 30% perlite should be used (more or less perlite, I just grab a few huge handfulls and mix it in, works for me)
As for the price I was paying less than $10 in Humboldt, but thats where the shit comes from so it's cheap all over the place. But now I live near seattle, and was researching where to get it, Online had some fucked off prices, So I started calling hydro shops and one had it for about $15/1.5cubic feet, so your best bet is a hydroshop if thats a possibility.
I hope any of this helped.
peace