Your GPTDash license is a one-time purchase. Separately, because GPTDash uses your own OpenAI API key, OpenAI bills you directly for the AI usage on your account.
For typical writing tasks this is very low, often a fraction of a cent per task. Images cost a little more. OpenAI sets and changes these prices, so check their current rates at openai.com/api/pricing.
Tip: you can set a monthly spending limit in your OpenAI account's billing settings so there are never any surprises.
GPTDash itself doesn't charge per use once your own key is connected.