Question

What amount of money was offered to Mexico to obtain western lands?

Answer 1

The United States also paid $15,000,000 ($482 million in 2016 dollars) for the land, and agreed to assume $3.25 million in debts to US citizens.

While technically the territory was purchased by the United States, the $15 million payment was simply credited against Mexico's debt to the U.S. at that time.