since the hotel car comes out to the airport empty they are charging you a typical taxi round-trip fare.........easy/cheaper just to get a taxi one-way at the airport which averages $60, but get a phone number for the taxi to take you back to the airport otherwise you'll get the hotel car ripoff fare........
GRU was talking about implementing a flat fee GRU-CBD but that was 2-yrs ago and don't know if they actually implemented it........