There is a "Cost Estimating Documentation Checklist for ACAT I, II and III Cost Estimates" included as Attachment 3 to AFI 65-508 Cost Analysis Guidance and Procedures (https://static.e-publishing.af.mil/production/1/saf_fm/publication/afi65-508/afi65-508.pdf)

This checklist, and MIL-STD-881F Work Breakdown Structures for Defense Materiel Items (https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=36026), should support BOE development.

There is also a Cost Estimating Guidebook developed by the Joint Space Cost Council. This document is specific to space acquisition, but could be a source of useful ideas.