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I think the cost of Facebook and Google ads blindsides many founders who take them for granted as a distribution channel when planning their startups, particularly founders who have no experience with them, or (even worse) founders who used them several years ago when they far cheaper and are unaware of how the landscape has since changed.

You should assume they will simply not be available to you for planning purposes. Occasionally you might stumble across an untapped niche where it is still possible to run ads at a cost permitting reasonable ROI but in my experience that is now the exception rather than the rule. If you do find such a niche it is more likely to be on Google, since their keyword-based model permits such niches to theoretically exist, whereas Facebook is an all-out competition by all advertisers for the same newsfeed slots.