A custom steel man cave generally costs 30–60% more than a prefab kit because it includes unique layouts, premium finishes, and extra design work. Prefab kits start around $15–$25 per square foot, while fully custom builds with insulation, interior finishing, and specialized features can range from $40–$80+ per square foot. Choosing a hybrid option—standard shell with selected upgrades offers a balance between affordability and customization.