Cloud cost surprises usually come from egress, idle resources, and the third-cheapest-instance-type-multiplied-by-thousands. A 30-minute estimation exercise saves the 'why is the bill 4x?' meeting.
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Compute is rarely the surprise
Instance-hours scale linearly with traffic and are well-priced. Use the calculator. Right-size after 2 weeks of production data (most teams oversize by 30-50%).
Egress is the trap
Cross-region: $0.02/GB. Internet egress: $0.08-0.12/GB. CDN re-egress: less but still pay. A 1 TB/day public-facing service is ~$3-5K/month in egress alone. Estimate egress first; it's usually the largest line.
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Storage and managed services
S3: cheap. Multi-AZ database (RDS, etc.): 2-3x single-AZ list price. Managed Elastic/Kafka: 3-5x DIY. NAT gateway: $30/mo + per-GB. Each $50/mo line adds up to thousands.
Estimate egress first, compute second, managed-service premiums third. 30% error tolerable; 4x is a process failure.