One key is broken...

What about mapping another key to the "z" key, like cap locks key or left shift. Not sure if that's possible but there in the same place range and that could buy you some more time.

 

I am not sure what your desires and needs are.

I'll assume the most inexpensive is a priority, in order of lowest cost first;
1. Key mapper, as IngerMan suggested. Note: some have not been updated for Sierra yet (no idea of what OS you are using).
2. External keyboard (USB or bluetooth).
3. Replace rMBP top case.
4. Buy a new computer.

 

How mobile are you?
Is either a wired or a compact bluetooth keyboard doable?

 

Get a bluetooth or USB keyboard. Or, repair the laptop yourself by replacing just the keyboard. Part will be less than $50 probably and you just need to be VERY meticulous when doing the repair. You don't need a whole top case like they do at Apple.

 

They'll most likely be replacing the battery as during that repair. Keyboard, top, battery (and I think maybe even the trackpad too.)

Silly thing is, is that I think if you just get the battery replaced, that includes a top case replacement too. Maybe try to see if you can have the battery replaced for the normal fee, and then ask them, "hey, while you're in there replacing the battery, would you mind replacing the keyboard while you're at it? I'll pay for the keyboard."