Volume tiers look simple: promise more, pay less per GB. The interesting behavior is at the boundaries, where committing to more traffic than you have can reduce the bill.
The mechanics
A tier sets your per-GB rate based on committed monthly volume. Usage above the commitment bills at the same tier rate; usage below it pays the minimum. The rate applies to every gigabyte, not just the marginal ones, which is why crossing a threshold repriced your entire month. The step structure also explains why two honest quotes for the same traffic can differ: one seller checked the structures above your volume and one did not.
The crossover effect
Because the rate drops in steps, there are volumes where the next tier up is cheaper in absolute terms. At some volumes, paying for capacity you do not use costs less than paying a higher rate on capacity you do. A good quote checks every tier above your usage and picks the cheapest structure, not the nearest label. Our pricing estimator does that arithmetic live. Our own rate card demonstrates the effect plainly: at certain volumes, a petabyte commitment at $0.009 per GB undercuts a 750 TB commitment at $0.015, and our estimator says so out loud rather than hoping you will not do the division.
Tier design also explains an oddity buyers notice on our own pricing page: select 750 TB and the estimator tells you plainly that a 1 PB commitment costs less in absolute dollars. We could have hidden that inversion. We surfaced it as a tip instead, because a rate card that quietly hopes you will not do the division is a rate card training you to distrust it. Any provider whose tiers contain such crossovers, and most do somewhere, should be asked directly: at my volume, is there a higher commitment that costs less? The quality of the answer tells you about more than pricing.
Sizing honestly
Commit to your realistic floor, not your optimistic forecast. Monthly-rolling terms let you step tiers as reality arrives; long terms should be priced as if your growth curve might not. The minimum you are certain to use is the commitment that never embarrasses you. The step-down conversation is the neglected half. Businesses shrink, seasons end, architectures change, and a contract that only steps upward converts every downturn into overpayment.
In practice
Before signing any tier: compute the absolute monthly cost of your tier and the two above it, at your realistic volume. If a higher tier is cheaper, take it and enjoy the headroom. Then confirm in writing what rate applies above commitment and how stepping down works. Ten minutes of arithmetic, applied once, outperforms most negotiation theater.
Try the tier engine on the pricing page, or let the assessment size your commitment from real logs.