Cheap open-ended ticket to South America?

I’m planning on embarking on an extended travel/work adventure to South America with my significant other. We were planning on leaving California on New years eve (cheap one way tix) for Buenos Aires. However, I’ve heard waiting to buy a return ticket when you’re ready can be quite pricey. Any advice on how to go about buying plane tickets if we:
a. dont know when we are coming home?
b. dont know where we are going to be flying home from?

Thanks.