Answer: It depends on the platform.
For Google Ads, Microsoft Ads, Yahoo! Gemini (AKA Oath Ads), and LinkedIn Ads - we do the following:
1) Determine average CPC
2) Multiply by 10 (to factor in 10 clicks per day)
3) Multiply by 30 (average number of days in a month)
4) That will give us a minimum budget which provides enough click data to make educated optimizations.
E.g. $10 average CPC x 10 x 30 = $3,000 minimum budget
For Facebook and Instagram, it works a little bit different:
The Facebook recommended budget comes from conversions (leads/purchases). For full optimization, we look for 50 conversions a week. So if a conversion is $10, we’re looking at a $500/week or $2k/month budget. The more budget, the more room we have to test different optimizations to find what suits your business the best.
Of course, this is just a recommendation. Budgets can be more or less, and estimates are not always 100% accurate.