Could you add in the reservation rates they paid to the table?
SELECT T1.firstname ,  T1.lastname, T1.Rate FROM Reservations AS T1 JOIN Rooms AS T2 ON T1.Room   =   T2.RoomId WHERE T1.Rate - T2.basePrice  <  0
reservation rates paid refers to T1.Rate; joining tables Reservations and Rooms on Room and RoomId respectively; filtering where paid rate is less than the base price of the room;