The simplest report is 3-1-1-61  Job Balances.  It shows the current contract amount per job, what you have already invoiced to date, the unbilled contract, and the ending balance which is how much has been billed but not yet paid (including retention if you hold it)

Hi Brett. You may also want to look at the 6-1-7 Over/Under Billings reports. They are budget, costs, and billings based. You can refer to Knowledgebase article 33371 to see where the numbers come from.

this is perfect and exactly what we are looking for.  thanks!