I disagree. It costs almost nothing (gas for driving?) to search for some leftover plywood or particle board in the trash near construction sites or at the curb in suburbs (it could be harvested from old dumped furniture as well). A bit of sawing and sanding and the shelf is ready. Instead of the wall mountings, one can just stack these makeshift shelves, using cardboard boxes from the gear (opening facing to the user) for sidewalls. Secure a proper sized piece of plywood inside the box to make it even sturdier.

In one of my closets I have a 1/2" particle board cut to the perimeter of the closet and freely sitting on top of the old computer desk at the center, which is just 1/3 of the closet's width and 3/4 of its depth - perfect shelf for $0. The additional bonus - you are not restricted with standard shelves sizes. I've easily made an opening on the side of that PB shelf for the vacuum cleaner handle, as it's higher than the table, but used to be stored in that closet before the "upgrade" (my DW insisted it'll stay there) - no problem.

By the way, in clothes closet it is often possible to simply sort long dresses/pants and short shirts/jackets by the length, you will be amazed how much space can be found below. One of the hanger tricks to save space in such a closet even more is to hang some of them over the lower bar of another hanger. I have hungers with the special eyelet exactly for that, as well as special vertical hangers accepting up to 6 regular hangers on top of each other - they work very well and I'm sure one can just jig saw one out of a piece of wood for free.