Steel trusses are more expensive, BUT, you typically get a longer span between them. Most steel trusses are 10ft spacing, whereas wood is typically 2ft to 4ft range I believe. That also changes the construction. With steel, you run 2x6s truss to truss and typically mount a tin roof directly to it. With wood you put down plywood/OSB and shingles, or purlins and tin. Gotta look at the whole cost. When I built my shop, wood trusses and tin roof would have been the cheapest option if buying new. I found a good deal on some steel trusses on CL so that was the way to go. Also, you get more clear overhead space with steel trusses, which was a major factor for me.