reddit Premium costs $6/month and eliminates ads. That's an upper bound on how much reddit values serving ads to users. Include an API key with reddit Premium that users can plug into the app of their choice. reddit gets the value per user, gets the analytics per-user, and it's hard to complain about price gouging given this was the previous price for ad-free service.
Reddit makes money from ads having reach and they're hoping that only a small percentage get premium so that they can continue to make money off of ads.
Why would they reduce the reach their ads are having by making it easy for people to opt-out?
Their API pricing probably reflects the money they are losing by not getting the users on their own platform and showing them ads.
Their hope is that people would switch over to the main app so that their advertisers can pay them more.