50% to 66%? You never see such massive conversion rates in anything on the web. It would be closer to 3% to 5%. That leaves $250,000 per year. Most of those 500,000 subreddits are not overly active. 450,000 of those are so unused they're not even candidates for subscriptions.
In the process, that tiny $250k you yield, would destroy Reddit by shutting down at least 95% of all subreddits.
3 - 5% is all you need to keep half the subreddits alive. If 3 - 5% of the users are willing to pay then about half the subreddits will stay alive since, if the owner is unwilling to pay, chances are some user of that subreddit is willing to pay.
In the process, that tiny $250k you yield, would destroy Reddit by shutting down at least 95% of all subreddits.