Probably they should have found a way to require third party apps to display their ads.
The solution is only allowing 3rd party access to Reddit subscribers, to avoid the need to monetize through ads. If Reddit is asking 12000$ for 50M requests, then a regular Reddit user (344 reqs/day) would be paying around 2,5$/month, which seems more reasonable than putting the burden on the 3rd party developer.
Which is less than I pay for premium. If more people would just, I dunno, pay for services they use regularly, Reddit wouldn't have a problem.