According to the PMBOK Guide, a project budget is the authorized amount of funds allocated to the project.
The project budget is derived from the cost baseline, which is the aggregation of the estimated costs of individual activities or work packages. The project budget should reflect the project scope, schedule, resources, risks, and quality requirements. A project manager should review the base budget model with the team leaders in each country to establish an independent budget for each phase when delivering a highly customizable software solution in multiple countries with significantly different cultures. This will help to account for the variations in the project requirements, constraints, assumptions, and risks in each country and phase, and to ensure the accuracy and adequacy of the project budget. Reviewing the base budget model with the team leaders in each country to establish an independent budget for each phase is the best action that the project manager should do next in this situation. Overestimating costs and underestimating revenues to include the impacts of different cultural elements (B) may not be realistic or reliable, as it may create a false sense of security, waste resources, or reduce profitability. Applying the company's operational budgeting model may not be suitable or applicable, as it may not capture the specific characteristics and challenges of the project.
Asking the project management office (PMO) to review the draft budget (D) may be a possible action, but not the next one, as it may be premature or ineffective without reviewing the base budget model with the team leaders in each country first. References: PMBOK Guide, 6th edition, pages 235-236, 260-261.