[fragmede]

Don't be penny-wise and pound foolish though. Let's pretend I take a month to design, pick, configure, operationalize, and integrate with the project, a pub/sub bus that will cost $0 while it's not running by developing it as VMs on hardware I own. A hypothetical alternative costs $x/month for the first Y billion messages. If I'll never exceed 1,000 messages/month, having a hard rule to avoid running costs would argue for taking a month to build such a system.

A more pragmatic approach involves examining X and Y before deciding not to pay any money. (They're also different for everybody, and it also depends on your financial situation.) If X is less than the cost of 1 coffee from a local coffee shop, I personally would pay for it rather than spend the month building my own $0 monthly unused cost pubsub bus for 1 month.

(Time it takes to implement is a 3rd variable, Z, though I'll note that estimating software project is notoriously difficult to get right.)