How much should you spend on marketing?

I often get asked by the Business Owners how much should they spend on Marketing. Some say 10% of your budget, and some say 20%. To me, as soon as this question gets asked, I understand that most probably, your marketing is not working properly. Why? 

Because in the digital age we live in, you should measure your marketing activities and know exactly how much money you earn by spending 1$ on marketing. And if you know that every dollar you pay is bringing you 1.2$ back, then the question of how many dollars do you want to spend to buy 20% more dollars becomes meaningless. 

Of cause, you want to spend all your available money, and if you can borrow with a lower interest rate, you are probably willing to do this too. 

With this in mind:

How many dollars do you get back by spending 1$ on marketing?