Putting aside Unicorns and folks that have raised monster rounds, in my experience, most enterprise SaaS companies struggle to deploy more than $30k-$40k a month on Adwords effectively.
They may also be able to deploy a similar amount on Capterra and other lead-gen solutions.
SMB-focused SaaS companies often have better performance with Facebook in particular, and can often deploy more there. And paid social performance outside of Facebook (e.g., Twitter, etc.) for SaaS is generally poor and limited.
But overall, most B2B SaaS companies struggle to find enough “digital” places to deploy capital than have even mediocre (1x) returns.
The markets are often too nichey for B2C techniques to work as well, and there is too much competition for these tiny niches.
This is one reason you see so many B2B $$$ into field marketing (events, trade shows, etc.), paid webinars, funnel management, ABM, podcasts, etc. Those are more work than Adwords and Facebook ads. But they also are channels where seasoned marketers can deploy more capital in B2B than they can in most traditional “digital” channels.