My mini cooper came with two of those 250$ electronic keys and 1 plastic spare. I gave the other key to my mom for safe keeping and I keep the plastic spare in my wallet. I figure if I ever lose both my car key AND my wallet, I'm going to need someone to bring me the key anyway, so it might as well be with the 1 person I can trust/rely upon.
edit: fucked up thing is the battery in the electronic key gets recharged by being in the ignition, so if she goes an extended period of time without using the key, the remote functions don't work properly 😛