That'll be a lot cheaper than a rental. And the OP already stated it didn't need to be a high-end bike. You could buy that and give it to someone on the way out, or just leave it unlocked. It'll get "stolen" quickly enough.
I've done the Metro from Santa Monica to DTLA, Hollywood and back, and all I can say is YUCK. There were so many druggies and homeless on it. As a New Yorker I ride the subway here all the time and never encounter that many.