Add cost information before openai requests

[agamm]

https://www.npmjs.com/package/openai-cost-calculator

This should be added after/with: #11

[elie222]

If we want to log the actual cost after its been completed then this is some code to help:

// returns cost in cents
function calcuateCost(
  model: AIModel,
  usage: {
    prompt_tokens: number;
    completion_tokens: number;
  }
): number {
  let costPerCompletionToken: number;
  let costPerPromptToken: number;

  if (model === "gpt-4") {
    costPerCompletionToken = 0.003;
    costPerPromptToken = 0.006;
  } else {
    costPerCompletionToken = 0.00015;
    costPerPromptToken = 0.0002;
  }

  return (
    usage.completion_tokens * costPerCompletionToken +
    usage.prompt_tokens * costPerPromptToken
  );
}