Dont skimp on your craftsmanship either. So im all for repurposing materials to save money, especially these days, but just becsuse the materials aren't new or in perfect condition doesn't mean the final product can't be just as functional, reliable and long lasting when its designed and constructed properly.
Conventional metal roof, asphalt shingle, wood shingle, etc. Will provide protection for 20-50yrs reliably or fail in 5-10 if poorly constructed.
Below is a photo of a new goat shelter im currently in the middle of building. It is 9'x10' with 8ft walls sloping to 7ft. It will include 2 pass through standard 80" doors so that it can be entered from either pasture, as well as a louvered vent and 65"x 24" sliding glass window.
So far it is 99% salvaged material from other unneeded structures from the property, abandoned/discarded material (yes i dumpster dove for 2x4s), i found abandoned 6x6s and 4x6s in our woods from the previous owners to use as the base, or scraps leftover from other projects. The only thing i bought so far were the nails that i purchased at an estate auction. I got $500 + worth of nails and fasteners for $28. I have a life expectancy for it to exceed15-20yrs but i expect to grow the enterprise and be in need of expansion/replacement in 3-5 yrs. So i did not want to spend $2k on something i will likely replace with a much larger barn in the near future. That being said, i built it assuming it needed to last 15yrs +.
Its roof will be asphalt shingles left over from my chicken coop, sons clubhouse, and my fathers workshop roof that we replaced a few yrs ago. I keep EVERYTHING so should the need arise.....I still have tar paper, drip edge, and gutters from old projects.
Point is, conventional design and materials work great when constructed properly, but if you think outside the box, you can use almost anything and still end up with a strong reliable functioning structure.