Curious, why should losing a Prius fob be more expensive than losing your key for any other vehicle? Do you have to have a working fob in order to reprogram? Is there something particularly difficult about the Prius when there are no working fobs to start with? I've been running around with one fob for a while now, but I need to get a second and program it for an extra layer of security.
It is far easier to program a new fob when you have a existing fob. In a situation with no fob, you usually have to a seed reset which is done by Toyota or a "qualified" locksmith. There have been reports recently here on Prius Chat where a individual is being told they will have to change the ECU to the tune of $1800.00. It seems that the dealer who quoted the large figure either cannot do a seed reset or is not knowledgeable of the procedure. HELP!! LOST KEY FOB-VEHICLE NOT PROGRAMMABLE | PriusChat Either way, it is far easier to get a second fob, then to wait until you have NO fob or a NON working fob. I am sure other's more knowledgeable than myself will chime in, but the above is basically the answer to your question.
Every car owner should have two working keys for their car. Then if you lose one you still have a backup and can easily make more. If you lose your last key (on any modern car) you are hosed. It can cost $600-$2000 to make the car drive (depending on where you take it) not including the hassle of towing, down time etc. Why not spend $100-$150 and get replacement as "insurance".