Should I ask her how much for her rent and just plan around it?
This is an area that some men fine to be difficult, b/c (being men) they feel it's their place to help with W's finances. But change your thinking on this line. Remember that everything you do to "help" her live apart from you is enabling the S/A.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!