Milan is unfortunately an expensive city, for us locals as well. Planning to spend 15-30 Euro, means a pizza, small beer and coffee in a not Duomo area place - or not fancy touristy place.
Two places off my head that fit the above are Meucci in Via San Giovanni sul Muro or Superpizza in Via Sabotino. Both are cheap places with great pizza and little else.
In my experience, when tourists ask for non touristy places where us locals go they end up being disappointed because what they really want is not that