This is always a difficult question to answer. If there's only one operation the bill for the operating room and anesthesia is less. If you have one hospital stay and replace the knees four to seven days apart, then there are two operating room and two anesthesia bills to pay.
Either way with a single hospital stay you'll save money if you are there fewer days in the hospital when compared to having each knee done one at a time. Patients having both knees replaced four to seven days apart but during a single hospital stay are there an average of nine days. This is four days longer than when both knees are done at one time.
The real determining factor is whether or not there are any problems after surgery. Complications can be minor or major. Minor problems include blood clots, bladder infections, or heart palpitations. Major problems can range from blood clot in the lung, heart attack, and even death. With the exception of death, any of these can prolong your hospital stay and increase the total cost.
The best way to get an estimate of the costs is to talk with the surgeon's and the hospital's accounting office. At least you can find out how the costs compare given the average patient with no problems afterwards.