Project Management: Developing a Solid Budget Estimate
Use expert judgment, meaning one or more experts are solicited to analyze project costs. This method is most often used when no prior data is known to exist.
Analogous estimating uses actual costs or data from prior projects to develop an estimate for the currently planned one.
Bottom up requires detailed accounting for all project components in more "real," concrete detail than the other methods.
"PERT" (Program Evaluation and Review Technique) estimates allow project managers to consider "most likely," "optimistic" and "pessimistic" scenarios in coming up with an estimate.