i spend 19$ a bag to fill 7 of my 8 pots so once in while i need to buy two bags.
i'd say soil roughly cost me 23$ a grow when i m producing up to 200gr per grow, so not a huge cost in comparaison to what it would cost to buy 200gr of weed. and this is also true when i have added the nutrients cost and electric bill.
a grow cost me roughly 150$ all included for up to 200gr of weed, i'd say that's cheap.
I've had good results using Kellogg brand soils from Home Depot. They market an outdoor potting mix and an indoor potting mix, the former a bit cheaper but containing manures, if you don't mind the occasional gnat.
Either way, I mix it, according to package directions, with Kellogg Organic Plus Tomato Fertilizer. I dust that with dolemetic lime and mix that , two to one, with perlite.
I'm a small, under-the-radar grower for personal consumption and while I could probably use it twice, I spread the used medium over outside stuff. There is enough left in the medium that the outside stuff likes it.