The first thing to do is investigate what you’re going to get out of what you spend.
Is your site well optimised in terms of keywords already, so all you need to do is hire a blogger to provide a steady stream of content for your blog or resources section?
Do you need to develop your social media presence?
Is your website not performing at all and you need the help of a digital marketing agency to help you revamp your online presence?
Consider what you need, and then think about what is realistic from an expenditure point of view for your business. If you manage to find a great digital marketing partner, you will see your investment repaid many times over. Keep in mind that cheap digital marketing solutions will usually have a catch, which is often that black hat SEO practices are being used.
In terms of how much of your business revenues/cash at bank you should be spending, look at what you can afford to commit to digital marketing without putting other areas of your business at risk and taking too much cash flow out of the company, all the while considering the potential returns.