Short answer: Once a key goes missing, you buy one either way. Bought with a working key in hand, a spare is just the key, from $129 on a Toyota chip key. Bought after losing the last one, it is that key plus $60, paperwork and a wait. Through a dealer, a flatbed adds $125 to $200, sometimes with days of waiting.
Many drivers put off the spare because the car starts fine today. But a second key never gets less expensive than on a day the first one still works, and the gap is easy to put in dollars.
Below, that math runs on figures from our price list and the dealer quotes beside them. After that: the households where one spare falls short, the right place to keep a spare, and the reason a copy from a key counter does not count.
The math on one car, both ways
Take a Toyota that uses a plain chip key. A spare made today starts at $129 from us. Suppose the original goes missing two years from now. With the spare in the drawer, you keep driving and replace the lost key when it suits you, from that same $129, and the car is never out of use.
Without a spare, that same loss puts you in all-keys-lost territory. The blade is cut from a code tied to your VIN, the car's immobilizer is rebuilt around a fresh chip, and the bill starts at $189. You also dig out paperwork, because a key made from the VIN requires ownership papers whose name agrees with your ID, and the car stays put until the van reaches it, usually within 45 to 90 minutes if the call lands inside open hours.
At a Toyota dealer the same loss runs further. The key is typically quoted around $250, a flatbed to the service bay adds $125 to $200, and stores that bill programming as its own line add another $100 to $180. Before tax, that is $375 to $450 without the labor line, and an out-of-stock key leaves the Toyota parked at the dealer for up to three days.
What the spare really buys you
Notice what the math does not say. The spare does not save the price of the key, because after a loss you buy that key anyway. What it saves is the $60 a no-key job adds, the hunt for paperwork, and the hours the car sits wherever you left it. Against the dealer route, it also saves the tow and the days.
Put another way, a household that never loses a key spends that $129 on a backup it never uses. A household that does lose one pays for a key either way, and the spare decides whether $60 and a wait ride on top. At the dealer, the gap between buying a spare and replacing a lost last key is mostly the flatbed and the wait.
Push-to-start cars make the gap wider
Smart keys raise both sides of the ledger. A Ford smart key starts at $239 from us while another fob still works, or $299 once none is left. Ford stores typically ask about $450 for that fob, and a car with no fob gets towed in, so the no-spare dealer route lands at $575 to $650 before tax or a labor line.
European cars stretch it further. A BMW smart key, the kind used on 2010 to 2018 models, starts at $399 here as a spare and $459 with none left, where BMW dealers ask around $750 before any tow. For most Mercedes-Benz cars built in 2014 or later, Mercedes alone supplies the replacement, so the spare in your drawer is the one fallback that skips the dealer.
When one spare is not enough
A spare is a key nobody carries. When two people share one car and each keeps a key on their ring, both keys are in daily use and the car has no spare at all. That household needs a third key in the drawer, and the math above applies to the third key exactly as it would to a second.
The same goes for a car handed to a new teen driver, a car one partner takes to work while the other borrows it on weekends, and a car that came home from a private sale with a single key. Count the keys that leave the house each morning, then add one that stays behind. Two cars means two spares, since a Honda chip key does nothing for the Kia parked beside it.
Where the spare should live
Inside the house, in a spot every driver knows, off the ring the main key lives on. A drawer near the door works. The glovebox does not: a spare locked in the car is no help in a lockout, and on many push-to-start cars a fob inside the cabin lets whoever is sitting there start the engine. A magnetic box under the bumper has the same flaw with extra steps.
If you commute far from home, a second spare at the office or with a relative across the county can spare you a long round trip. Wherever it lives, run the engine on the spare every few months and press each button. A fob that has sat unused for years may want a fresh coin cell, which a parts store sells for a few dollars.
A key-counter copy is not a spare
The immobilizer keeps its own list of keys it will start for, and a key earns a place on that list only by being programmed to the car. A blade copied from yours at a hardware store turns the locks and the ignition cylinder, but on a chip-key car it is not on the list, so the engine will not run on it.
A few key counters offer chip cloning for older keys. A clone starts the car by imitating your original, so it never gets an entry of its own. Once the original is gone and you have it erased so a finder cannot use it, the clone is likely to quit as well, so the money spent at the key counter buys no lasting backup. A programmed spare car key is registered in its own right.