Adjusted OpenAI cost calculation (#6798)

Added parentheses to ensure the division operation is performed before
multiplication. This now correctly calculates the cost by dividing the
number of tokens by 1000 first (to get the cost per token), and then
multiplies it with the model's cost per 1k tokens @agola11

langchain/callbacks/openai_info.py

@@ -96,7 +96,7 @@ def get_openai_token_cost_for_model(
             f"Unknown model: {model_name}. Please provide a valid OpenAI model name."
             "Known models are: " + ", ".join(MODEL_COST_PER_1K_TOKENS.keys())
         )
-    return MODEL_COST_PER_1K_TOKENS[model_name] * num_tokens / 1000
+    return MODEL_COST_PER_1K_TOKENS[model_name] * (num_tokens / 1000)
 
 
 class OpenAICallbackHandler(BaseCallbackHandler):