It is important to track the reason behind any major changes to the cost of a project. Project costs can be affected by time changes, scope changes, price escalation factors and many other reasons.
Recommended sources: Project management timelines, budget documents, quantity surveyor or other technical adviser reports.
Timing of data input: Ongoing throughout project.
Issue: The table is a fairly high-level snapshot of changes and their impact. The comments section is where explanations can be provided.
Issue: It may not be possible to translate time changes into a $ impact (e.g. additional project management costs may be identifiable but not opportunity costs). In this case note the explanation for the major time changes in the table or comments section.
Issue: Scope changes and price changes may impact either or both of capital or recurrent components of the project budget/cost. Where it is possible to separate, then there is space to do in the comments section.