Costs? These materials are easily found roadside or elsewhere. The blocks have chunks missing or are uneven, so a matchbook or folded cardboard helps level out the wobble for the weather-soaked grey plank.
I know as I’ve done just that.
Also a lot of construction sites are kind enough to leave a pile of free construction materials somewhere on the site, like they are saying, “we have extra so help yourself! 😁” I try to only help myself at night, because I’m considerate enough to avoid getting in the way of the actual construction.
You should get rid of things with a “modicum” (tiny amount) of useful value. Your space and mental energy is used up just having it there.
You have to keep track of it or it’s not useful at all. That takes energy that you could use for something else. Restfully relaxing in a clean room is worth a lot.