How To Set A Marketing Budget From The Surprisingly Helpful SBA.gov

Many businesses allocate a percentage of actual or projected gross revenues – usually between 2-3 percent for run-rate marketing and up to 3-5 percent for start-up marketing. But the allocation actually depends on several factors: the industry you’re in, the size of your business, and its growth stage. For example, during the early brand building years retail businesses spend much more than other businesses on marketing – up to 20 percent of sales.

As a general rule, small businesses with revenues less than $5 million should allocate 7-8 percent of their revenues to marketing. This budget should be split between 1) brand development costs (which includes all the channels you use to promote your brand such as your website, blogs, sales collateral, etc.), and 2) the costs of promoting your business (campaigns, advertising, events, etc.).

This percentage also assumes you have margins in the range of 10-12 percent (after you’ve covered your other expenses, including marketing).

If your margins are lower than this, then you might consider eating more of the costs of doing business by lowering your overall margins and allocating additional spending to marketing. It’s a tough call, but your marketing budget should never be based on just what’s left over once all your other business expenses are covered.
— SBA's website http://bit.ly/2hQyUwo