If you're focusing on inbound techniques like SEO, social media, and content creation for a preexisting website, the good news is you don't need very much budget at all. With inbound marketing, the main focus is on creating high quality content that your audience will want to consume, which unless you're planning to outsource the work, the only investment you'll need is your time.
With offline marketing, it's very difficult to tell how people are interacting with your brand before they have an interaction with a salesperson or make a purchase. With digital marketing, you can identify trends and patterns in people's behavior before they've reached the final stage in their buyer's journey, meaning you can make more informed decisions about how to attract them to your website right at the top of the marketing funnel.
Now imagine you had that brochure on your website instead. You can measure exactly how many people viewed the page where it's hosted, and you can collect the contact details of those who download it by using forms. Not only can you measure how many people are engaging with your content, but you're also generating qualified leads when people download it.