I have some 30 10 gal+ containers that I set out each summer, for annual flowers, egg plant, herbs and so on. The cost to refill them every year would be prohibitive. I started with two and filled them with the least expensive generic potting soil I could find, usually from Wally World. In the spring, I would dump the old potting soil out on the cement porch, and mix in half half with finished compost, and then I would have more than enough to fill those two pots, so I'd go and get a few more containers, and another sack of new cheapo potting soil, and sort of expand the operation until my wife said 'nuf is 'nuf.

The organic matter breaks down over time, but the vermiculite and the perlite stay.

So now I just dump everything out and mix it all up with compost, and put it back in, and fill them up to the brim.

Any potting soil you buy will be either peat moss, finely gorund bark, or coir, or a combination of these things. All of these tend to be hydrophobic, water repellent, and need to be soaked with water (hot water absorbs better) at least 24 hours before putting in pots. Once in containers you need to be very careful about watering since any of these that are allowed to dry out will again be hydrophobic, although soaking them in a tub of water (not hot water this time) can get them to absorb water again.