It depends where you are, Rachael. Peter Mac had a 'slush fund' that they used to help me with the cost of a PET scan--the prospect of brain mets accompanying my recurrence was the reason I got a bit of a boost as I simply did not have the $900 it was going to cost. MRI funding is a bastard situation if you already have BC; the only way around it is to go 'looking' for something else that is funded that just may be in the same place as your breast cancer concerns That requires a bit of finangling and a medico who is willing to bend the rules a little. Most won't which is completely understandable .
It's distressing to know you can't afford some of these things, but the science isn't that convincing in many cases anyway. There are numerous women here who have had tumours discovered post surgery that did not show up on an MRI--that is particularly true for those of us blessed with lobular carcinoma.
Paying for bone density scans pisses me off too, but I'm stuck with that until I'm porous enough to qualify as rooted.