SingleCare offers a coupon that you might be able to get 6 doses for about $250. There are different discounts by location.
https://www.singlecare.com/prescription/fulvestrant?,6%2C%205ml%20of%20250mg/5ml%20Syringe%20of%20Fulvestrant.
AstraZeneca has a webpage with supports for lowering costs for fulvestrant. Lower drug costs are not available for Medicare patients, but they have links for outside organizations that MAY help.
https://www.myaccess360.com/patient/faslodex-fulvestrant/patient-affordability.html
It looks like Medicare would maybe pay for fulvestrant if you had the injection in a hospital. These days many clinics bill like they are a hospital so an out patient clinic may qualify. I’d suggest talking to the insurance clerk at your medical facility. They often know what hoops to jump through to get something paid - because they’ve been doing the paperwork for it with other patients. Doctors often aren’t aware of what their own insurance staff handle.