A bag of packed red blood cells costs around $400 at my hospital. If blood is obtained from a donor cat and given to your kitty, the cost may be a little less, but still very close because there is alot more testing and screening that is necessary. The costs of the hospitalization, i.v. catheter and constant monitoring are often factored in, and these numbers will wary from city to city.
Blood transfusions are very expensive, and need to be monitored extremely carefully. Most daily veterinarians are able to perform this service, but many do not because of the long list of possible complications and the extensive testing that is necessary for both the donor cat and the cat receiving the transfusion. The cost is high because this is life saving medicine in a critical cat.
I hope that your kitty does well. If you have other questions, please don't hesitate to ask. If this has been helpful, please hit the green acept button.