Estimate how much...
Independent estimates are one of the tools and techniques for the PMI process to conduct procurements. These estimates can be made by the procuring organization or other organizations such as professional estimators for comparison to the prospective seller's quotes. Significant differences in estimates should be investigated for underlying causes to ensure they meet the requirements and do not include unnecessary components.
Related: risk, procurement, procurement negotiations
Section 12.2.2.3 of the PMBOK, 5th edition, describes independent estimates.