Maybe this would make more sense?
TotalRateAllPositions: (IIf([CABIHours]>8,(500+([CABIHours]-8)*93.75),[CABIHours]*62.5),IIf([ProjectManagerHours]>8,(576+([ProjectManagerHours]-8)*95),[ProjectManagerHours]*72), IIf([AMSHours]>8,(576+([AMSHours]-8)*95),[AMSHours]*72))))
I keep getting a coma...