In case you rent an apartment and just by chance lost a door key, the property manager might charge you a hefty amount for a replacement key. To avoid that, you might want to try the following places to make a duplicate door key.
home improvement/hardware department
said to have self serving key making machine
The cost is fairly low, for $1 to $2 you get a basic door or car key copied at most local hardware stores. However the copying is done by a store employee who may or may not have the proper training compared to a locksmith.
Also, even if the key has "do not duplicate" on it, you can still make duplicates. There's no law that says you can not make duplicates. It's more of a security measure.