More general information about rate limiting and the “cost” of API calls can be found in our API Rate Limiting documentation. To be very clear, there is no monetary value associated with an API call. The API call “cost” is just put in place as a rate-limiting measure to protect the platform.
On our basic plans, the cost of a DPU will be roughly equivalent to $0.20, yes. However you should contact a member of our sales team to discuss your requirements; they will work with you to build a package with pricing that best fits your needs.
There is no concept of “credits” for Facebook Topic Data access. In order to access Facebook Topic Data, you should look to contact one of our data partners.