Reddit should realistically charge for lost revenue rather than lost expenses. Instagram makes $50B annual revenue on 2 billion users, or around $25/user/year. I'm certain that Reddit is making nowhere near that per user, but they're probably pricing their API along their most optimistic projections.
I can't find the source but in some of the communication Selig released between him and Reddit a few days ago, Reddit themselves confirmed that their pricing is based on lost oppurtunity cost per user, not just expenses
I don’t really think of any number, because that would be silly. None of us know the answer nor can we confidently estimate what it is. Therefore any attempt to justify or argue costs by us is useless.