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Third party apps were either free or had a single one-time fee (typically less than $10), or had tiers for both. The most popular app was Apollo, which had a free version, a $5 "pro" version with extra features, or an "ultra" subscription that was $1.49 per month "due to options within it having ongoing monthly costs to me [the lone developer]".
Here's what the Apollo develop had to say specifically about the sudden fees to use the API: "Apollo made 7 billion requests last month, which would put it at about 1.7 million dollars per month, or 20 million US dollars per year. Even if I only kept subscription users, the average Apollo user uses 344 requests per day, which would cost $2.50 per month, which is over double what the subscription currently costs, so I'd be in the red every month."