Building a Cloud Cost Calculator with the Azure Retail Prices API

Why First-Party Pricing Calculators Fall Short

The Azure Pricing Calculator is excellent for static estimates. It falls short the moment you need to answer questions like: "If we move from 8 D4s_v3 nodes to 6 D8s_v4 nodes and shift workloads to East US 2, what's the delta?" Or: "What's the 3-year reserved instance break-even point for this specific SKU in this specific region?"

For the cloud infrastructure consultancy work at Foetron, we built a custom pricing engine that hits the Azure Retail Prices API directly and layers business logic — block size constraints, reserved instance amortisation, licensing included/excluded comparisons — on top.

The Azure Retail Prices API

The API requires no authentication. Every SKU, every region, every pricing model is queryable:

Response shape:

Architectural Constraints That Drive Cost: Block Size

Block size is the single most underestimated cost driver in cloud architecture discussions. A workload that perfectly fits a D4s_v3 (4 vCPU, 16 GB RAM) will cost roughly the same as one using 20% of a D8s_v3 (8 vCPU, 32 GB RAM) — if the workload doesn't fit the smaller SKU.

The block size problem manifests when:

An application requires 18 GB RAM → must use a 32 GB SKU (D8s) → paying for 14 GB of idle RAM
A database needs 6 vCPUs → must use an 8-vCPU SKU → paying for 2 wasted vCPUs
A bursty workload needs 20 vCPUs at peak → over-provisioned to 32 vCPU SKU to handle the burst

Our calculation engine models this explicitly:

Reserved Instance Break-Even Calculation

The 1-year and 3-year reserved instance prices are in the same API (

). The break-even point in months:

For most D-series VMs in East US running 24/7, the 1-year RI break-even is around 7–8 months and the 3-year RI break-even is 14–16 months.

A Real Optimisation Example

A client was running 12× D8s_v3 VMs (8 vCPU, 32 GB) on pay-as-you-go for a web tier that needed 6 vCPUs and 20 GB RAM per instance.

The analysis:

| Option | Config | Monthly (USD) | |---|---|---| | Current | 12× D8s_v3 PAYG | $2,006 | | Right-sized | 12× D4s_v3 PAYG | $1,003 | | Right-sized + 1yr RI | 12× D4s_v3 Reserved | $604 | | Spot (non-critical tier) | 8× D4s_v3 Spot + 4× PAYG | $389 |

Outcome: $1,617/month saving (80.8% reduction) by right-sizing, moving to reserved instances for the base load, and using Spot for the stateless tier — all discovered via the pricing API before touching a single VM.