Last week I bought a 36″ wide interior fire-rated door from PSU salvage for $10. Cut in half that’s about 12′ feet of shelving for less than a dollar a foot. The only problem is that the core is chipboard and so you have to attach an edging.
But with a couple of brackets and conduit for a hanging rod it’s a solid shelf at a fraction of the cost of anything you can buy at the hardware store.