A Comparison of Budget Models
Fixed price. A fixed price is suitable when the scope of work, assumptions, data availability and client obligations are clearly defined. Costs can be planned at an early stage; however, changes outside the agreed scope must be managed via a transparent change request.
Cap or cost ceiling. With a cap, invoicing is based on actual expenditure, but only up to an agreed limit. This model is suitable for projects where the overall direction is clear, but where individual work steps still involve some uncertainty. It is crucial to report at an early stage when and why expenditure is approaching the upper limit.
Phase budget or target corridor. Longer or dynamic projects can be broken down into clearly defined phases, each with its own budget and approval points. A target corridor can also provide a realistic range. This enables senior management to decide on scope, priority and the next investment after each milestone.
Retainer or performance-related component. Recurring project management can be remunerated via a retainer; a performance-related component is only added in suitable circumstances. Both models require a clear description of the services included and must not obscure which special tasks are to be charged for separately.
No model can replace a clear statement of work. Even a fixed price becomes unpredictable if assumptions and obligations to cooperate are missing. Conversely, time-based billing can be easily managed if a robust budget, warning thresholds and regular forecasts have been agreed.