The most common mistake in reducing cloud costs is looking only at the "total." When monthly bills drop from $100k to $90k it looks like success, but if traffic fell 50% in that period, you've actually worsened unit cost. Cost Optimization's real metric isn't total spend but unit economics (cost per request, per user, per transaction). Sustainability translates this same thinking to carbon—the goal is reducing "carbon per useful work," not absolute emissions.