In Estimates (and Invoices and Sales orders?) add TAB with SPERADSHEETS. With this option, we can have all cost calculations used to create current transaction (Estimate, Invoice, Sales Order). When we need create similar transaction, then we can copy old one and modify spreadsheet to get new costs.

At this time we can connect external spreadsheets, but when we copy transaction and open connected spreadsheetand save new data, we will lost old calculations (on old documents we will have connected spreadsheet with last saved datas).