Who ever came up with that price is looking for short-term profits over user happiness and long-term growth.
An app that uses reddit is not a competitor to reddit, it's a client of reddit. No definition of "anti-competitive" applies here.
Now, whether this constitutes “anti competitive” in the legal sense is probably not going to fly in court: it’s unlikely Reddit can be compelled to offer an API at any particular price. It’s their service, they can do what they want with it. Rather, it’s a lesson that third parties should not be developing clients for other company’s services, as it is building a foundation on quicksand.