Founders usually budget for the build and forget the running costs. For a typical single-platform micro-SaaS with a few hundred paying customers, the first year of infrastructure lands between $1,200 and $3,000 — much less than most people expect, and structured differently.
The fixed costs are small: a managed database, a container or two, object storage, a CDN, error monitoring, transactional email, and the developer programme fees for each store. Together that is usually under $150 a month at small scale.
The variable costs are where attention belongs. Payment processing takes its percentage of every transaction. Any AI feature bills per request. Anything that stores user-generated media grows forever unless somebody sets a retention policy. We model all three during scoping and put a ceiling on the ones that can run away, because an infrastructure bill that scales faster than revenue is the quietest way to kill a small product.