Blindly picking a SQL DB (mysql/postgres etc.) is quite expensive from the get-go (a production ready mysql/postgres would cost ~30$/m).
Mongo costs ~$10/m (MongoDB Atlas), Google's Datastore is Pay as you Go (so your initial cost is close to $0 till you get paying customers), AWS's DynamoDB is similarly priced as well.
Sure, sadly all those noSQL solutions get really expensive as your usage goes up to normal non-webscale proportions, but at that point you have the $ to invest in a SQL solution.
The above was mentioned with bootstrapped startups/services in mind. Not your usual million funded valley companies.