The correct service for defining and tracking a financial plan and receiving alerts when expenditure thresholds are exceeded is AWS Budgets. This tool is specifically designed for budget management and cost tracking against user-defined criteria. AWS Cost Explorer is more focused on analyzing and visualizing spending patterns over time, rather than setting and managing budgets. AWS Billing Conductor facilitates the customization of billing data; however, it does not encompass budget setup or monitoring functions. The AWS Price List Service is simply for accessing the prices of various services, not for budgeting purposes.