Having built budgets at multiple SaaS startups, the honest answer is there is no stock answer. What's guided me time and time again is this principle: Budgets are a reflection priorities. If you approach with priorities first, and back in with budgets, you're more likely to see short-term successes that can be benchmarked and used to guide future investments.
For example, let's say your priorities are primarily lead generation and customer acquisition at this point. Using a small budget you could look at a few different channels such as PPC, SEO, retargeting, events, etc. At first you'll have to look at industry benchmarks to set some expectations around costs and expected returns. (Knowing these are not likely to be 100% in line with what you see.) Aspects like landing pages, content and collateral, etc. should be in the budget, but they represent the support for your paid programs, not a paid program itself. (For instance, a landing page is used to capture the lead, but the lead could have been sent to the landing page from a LinkedIn ad.)
You'll likely want to try at least a couple different tactics, measure results, and then invest more in what's working in the short term. But you can't just spread the money around as you need dollars and time to accurately capture what's working and what's not. And when measuring results, you need to look at more than the acquisition costs and number of leads, but also the velocity and quality of those returns. PPC might deliver a ton of leads, but it turns out your redesigned website is driving better quality leads that are converting into customers at higher clip. This provides the best view of where budget should be allocated.
Again, it all comes down to priorities, which can and will change. But these must represent what goals you have for your dollars. Otherwise you'll be bobbing for apples.