Marketing budget for a small business - where do I start?

In four steps: take annual revenue, work out your real margin, set a target in customers rather than in money, then split the budget across channels. A budget built this way survives a weak month, because you know what it buys.

Keep it monthly rather than annual - it's easier to manage and faster to correct. Hold it steady for at least three months or you'll never learn what worked. And count the work, not just the media: photos, copy, the website and the hours spent answering customers are part of the budget even when no invoice arrives.