What are variable costs?
Variable costs are expenses that change in direct proportion to output or sales volume: raw materials, direct labour, payment-processing fees, cloud infrastructure that scales with users, sales commissions tied to revenue. Variable costs are the building blocks of COGS and contribution-margin analysis.
Variable costs in different business models
- Manufacturing: materials, direct labour, energy used in production.
- SaaS: hosting (compute, storage, bandwidth), customer-support staff that scales with users, third-party API calls, payment-processing fees.
- Marketplaces: payment processing, transaction-based partner economics, fraud prevention.
- E-commerce: COGS of goods sold, fulfilment, shipping, payment processing, returns.
Variable costs vs. fixed costs
- Variable: rise and fall with volume; per-unit cost relatively constant.
- Fixed: insensitive to volume in the short run; per-unit cost falls as volume rises.
- The variable-to-fixed mix determines operating leverage and break-even sensitivity.
Contribution margin
Variable costs feed the contribution-margin metric: Contribution Margin = Revenue − Variable Costs. The contribution margin is what is left to cover fixed costs and then deliver profit. Pricing decisions, especially discounts, hinge on what they do to contribution per unit, not gross margin alone.
SaaS-specific watch-outs
- Cloud costs (AWS, GCP) are increasingly variable with usage: what looked fixed in 2018 is now usage-priced.
- AI inference costs scale with usage and can dominate variable cost for AI-native products.
- Payment processing (Stripe, Adyen) is a steady ~2-3% variable cost on revenue.
Do: instrument variable cost by customer cohort and product line; understand how it shifts with scale and product mix.
Don’t: assume variable cost per unit is constant indefinitely: AI inference, cloud egress and support load all shift with scale.
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