What are the expenses involved? It all depends on the program you are in. Some program will cover the veterinary cost only. Food, grooming, toys and all other the foster home will have to take care.
Most program will not allow you to choose the dog you would like to foster. How long you have to foster the dog will depend on when the dog will be adopted. Sometimes it will take as long as a year or two. Sometimes you will get to foster for two to three weeks only.
What it takes to be a foster carer? First you must be a dog lover. Experience in looking after a dog is also a must. Inexperience in dog handling will have difficulties in dog behaviour. You will certainly need to have patient and be totally committed than only fostering a dog will be very enjoyable and rewarding.