How to understand customer journey in a B2B SaaS mobile application? Without using expensive softwares like Mixpanel or Yourstory.

How I've understood customer journeys in the past have never actually leveraged a software tool to do so. We've started by mapping out the customer journey into normal stages: awareness, interest, consideration, preference, loyalty. Then across those stages, look to see what is the customer thinking, doing, experiencing and feeling across each. 

Fill all of that out into a matrix and from there you'll have a good grasp for what your customer is going through across the journey and that should help shape the assets you need to create or the campaigns you need to build to move customers through the customer journey towards purchase/loyalty. 

At my previous company we did this by interviewing internal stakeholders who were customer facing as well as interviewing customers when we could--so it was all internally executed without 3p SaaS. 

I'd start there and if there was still a gap in terms of what insights were needed, then I'd consider a software tool as a step two in the future.