Firstly, apologies for the long winded response, but this is not a straightforward subject !!
I am assuming that this question relates specifically to an assessment of prolongation and the associated People costs, rather than the question of PAYMENT for these resources, which is a different topic altogether, and depends upon the Main Option.
You state that PMIs have been instructed, which assumes that there have been notified compensation events. As a compensation event is not a risk that the Contractor has allowed for within his baseline programme, it will, consequently, have a disrupting influence upon the works, to a greater or lesser extent.
This disruption causes a 'resource thickening' issue whereby over time, supplementary resources are required to manage the works, although there is no delay to planned Completion, with the additional People required to deal with the consequent disrupting influences.
Clearly the cost of these additional People cannot reasonably, or contractually, be allocated to a single compensation event, so an ‘increasing’ cost assessment should be included in separate quotations to account for this. The Prices then includes allowances for an additional amount of People, beyond that at the contract date.
Where a 'resource thickening' assessment is made, it should, however, properly address the issues of; causation, remoteness, mitigation and contractual compliance, based on some considered and intelligent principles of how the amounts have been assessed. A common approach taken is a 'global claim' type assessment, which just assumes ALL additional resources are due to compensation events. Even though the calculation is a difficult one to assess, this approach is unacceptable. I note that some contracts now include a percentage addition, for this very assessment within Contract Data, to avoid the subsequent 'frank discussions' !!
This is not to say, however, that ALL of the additional resources on Site are specifically due to compensation events, although where there is a delay to planned Completion due to a compensation event, then ALL of the resources should be included in a prolongation assessment, where it is demonstrated that they are all required for activities which are critically linked to planned Completion.
Note that a quotation assessment should be based upon the following;
Total actual and forecast Defined Cost including the compensation event
Total actual and forecast Defined Cost NOT including the compensation event
I hope this helps.