Under the ECC core clause 52.1 “Defined Cost includes only amounts calculated using rates and percentages stated in the Contract Data and other amounts at open market or competitively tendered prices with deductions for all discounts, rebates and taxes which can be recovered deducted.”
So if you have tendered a relevant rate or percentage at tender i.e. referenced from Contract Data part 2, then that is used regardless of whether it is a high or low rate or percentage and whether and forecast or assessed retrospectively.
For other costs, its “open market or competitively tendered” and being a civil contract, proof is ‘balance of probability’ and not ‘beyond reasonable doubt’ as per criminal law. For forecasts therefore, I would have though an invoice or quote would be sufficient. For retrospective assessment, a receipt if you have it would be OK. Looking at bank account statements on a price based contract seems a bit over the top.
Note that clause 63.1 gives direction on when a forecast should be used as opposed to a retrospective assessment.
Lastly, if the PM really does think that the prices are too high and not at “open market or competitively tendered” rates or prices, he or she can market test them and use that rate providing it is on a like for like basis. I.e. if the Subcontractor is using brand new hired Equipment with a service level agreement where they will be on-site within the hour, the market rate will obviously be more than for knackered old kit which will be fixed in a week.