Add calculated total_cost to unified Callback Manager

[OwOHamper]

Checked

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Feature request

Would it be possible to implement how much the total cost was in the UsageMetadataCallbackHandler?

Similarly like in the traditional, for example openai callback (get_openai_callback) it would also calculate the price of tokens.

from langchain_core.callbacks import get_usage_metadata_callback
from langchain.chat_models import init_chat_model
from langchain_community.callbacks.manager import get_openai_callback


llm = init_chat_model(
    model="gpt-4.1",
)

with get_openai_callback() as cb:
    llm.invoke("Hello")
    print(cb)

with get_usage_metadata_callback() as cb:
    llm.invoke("Hello")
    print(cb)

python main.py
Tokens Used: 18
        Prompt Tokens: 8
                Prompt Tokens Cached: 0
        Completion Tokens: 10
                Reasoning Tokens: 0
Successful Requests: 1
Total Cost (USD): $9.6e-05
{'gpt-4.1-2025-04-14': {'input_tokens': 8, 'output_tokens': 10, 'total_tokens': 18, 'input_token_details': {'audio': 0, 'cache_read': 0}, 'output_token_details': {'audio': 0, 'reasoning': 0}}}

Motivation

I've found about unified callback handler that is work in progress from this pull request #30534. After migrating to this approach I've noticed the total_cost is missing, which means I am going to have to do all of the calculations myself.

Proposal (If applicable)

No response