What is the difference between value and cost in a monthly report?

These are the two halves of any honest cost report, measured in different ways from different places. Value is what the works are worth to you under the contract at the reporting date: not what you have invoiced or been certified, but your true entitlement, the measured work against the pricing document, variations at your honest current assessment, and any claim carried as the exposure it is. It is deliberately independent of the payment cycle, because a certificate that undervalues you does not make the job worth less, it just means you have not been paid for it yet. Cost is the mirror: everything the job has consumed to reach that position, labour, plant, materials, subcontractors, preliminaries and overhead recovery, including costs incurred but not yet invoiced, which have to be accrued or the cost line lies in your favour. What makes the comparison mean anything is a shared cut-off date: value to month end and cost to whenever the ledger last ran are two different jobs, and subtracting one from the other produces a margin that is pure fiction. Get both to the same day and the difference is the margin the job is truly earning; its movement month on month, explained, is the earliest warning you will get. Confusing value with what you have been paid, or cost with what you have been invoiced, is the commonest reason a report reassures a director right up until the job has already gone backwards.

Consider a small main contractor whose monthly board pack reports profit as certified income minus invoiced cost: it looks like a report and is worse than none. In a month where the certifier has been slow and cautious, certified income is low, so the job looks like it is losing money and the director panics over nothing. Next month a run of supplier invoices happens to arrive late, so invoiced cost is low and the same job looks wildly profitable, and the director quotes the next tender too keen off the back of it. Neither picture is real; both are artefacts of payment and invoicing timing. Rebuilt to value the works from the account to month end and accrue all cost to the same date, the margin barely moves between the two months, because the underlying job did not change, and the small genuine slip from an over-tender bricklaying gang shows through clearly instead of being buried under timing noise. The director stops reacting to the payment cycle and starts managing the job.