Well, you could try the passenger door...I'm sure it gets used less than the drivers for unlocking. That'll give you an idea if it's the key that needs replacing.
With that said, if a new key is difficult to turn in the ignition (i.e. you have to jiggle it to get it to work), you're going to have an extra master key because one comes with the ignition cylinder. When I bought a valet key, it was around $28, so, I'd expect something around that for a master key.