While all rescue organizations differ, all groups should provide all medical care that is needed for the pet. Many groups also provide pet food and supplies, so your costs should be minimal. Of course, any supplies you can provide will enable the rescue organization to help even more animals.
Your rescue organization should provide you with a point of contact for medical or behavior questions. They should also have clear instructions on what to do if you have any concerns about your foster pet as well as where you should take him or her if emergency medical care is needed. This is especially true for after hours and emergencies on the weekends and holidays.