According to a 2013 SMB survey, small and medium businesses spend an average of
$400/month on marketing and 46% of that is on digital marketing. In short, the average annual spend is $2,400.
This is much different from the question of how much they SHOULD be spending. The answer to that is much more complex and nuanced.
1. The amount you budget for digital marketing depends on your goals, your projected sales, and your disposable income. You need to be testing a variety of marketing channels in order to discover which channels produce the greatest return on investment (ROI) and then lean heavily into the channels which produce the greatest impact for you.
2. Another component to consider is the type of industry you are in. You need to consider the lifetime value (how much your customer will purchase over the lifetime of the business relationship) against the customer acquisition cost (how much you spend to make them a customer). If you have a high LTV to small CAC ratio, then you should be investing a greater budget in the marketing that helps you acquire them.