It's a common question and the answer varies a bit.
Whether the business is B2C or B2B, and the industry they operate in, also impacts how much they'll have to spend to reach their audience. B2B tend to spend more to reach their customers whilst Banking, Finance, Insurance & Tech sectors spend more than others.
How much is a new customer worth to your business?
The following graphic is a bit of a guideline of budgets by industry.
Example budget: XYZ Manufacturing had a gross revenue of $600,000 in 2019. They are B2C and looking to grow so they assign 13% of that to their marketing. 13% of $600,000 is $78,000.
So their marketing budget for 2020 is $78,000 or $6,500 per month.