I live in a WG 1 minute from
Sendlinger Tor u-bahn for €270 per month. I found it by going to the university (on
Leopoldstr.) and scouring their pin-boards for adverts. It's definitely possible to find a cheap place but you've gotta be willing to put in the hard graft.
If you need to find a place before you arrive in Munich then maybe find a not-so-cheap place with a short rental agreement and look for somewhere cheaper once you're here?
As mentioned before, there are lots of people in Munich looking for the same thing so much of the time it depends on luck

In any case, good luck with the search and welcome to Munich!