Gregory's ServerSomeone suggested on Metafilter... you could keep everything as-is (including Apollo subscriptions) and make the app available to the public to install.
But on first run, have them go get their own reddit API key (maybe even automate it). That way, you're not on the hook for back-end costs. You could also add screens to give users an estimate of their API cost (in case there's no free API tier).
If the user registers their API keys on your server, you could also make server-side calls on their behalf and keep notifications/etc. alive.
Crowdsourced API keys 😃