What will hosting cost as it grows?
The monthly bill at ten users, a thousand, and a hundred thousand.
I'm building <one sentence> with <stack>, hosted on <host>. Estimate what hosting costs at 10, 1,000 and 100,000 monthly users. Show your assumptions so I can change them. - Which of the host's limits do I hit first as usage grows? - At what point do I need a bigger plan or a different kind of hosting? - Which extra services will I end up paying for: database, email, storage, backups? - What's likely to cost more than I expect? Give me the number that matters now, and the one I should watch.
For most side projects the honest answer is "the plan you're on, for a long time". It's still worth knowing which limit you'll hit first.