You use the Cost Management dataset to calculate the cost before RI and Savings from RI
Using the following FlexReport query will give you the cost before RI and Savings from RI
{
"sqlStatement": " SELECT timeInterval_Month AS Month, SUM(CostInReportingCurrency) AS SUM_CostInReportingCurrency, SUM(Quantity) AS SUM_Quantity, ResourceName AS ResourceName, SubscriptionName AS SubscriptionName, ResourceGroup AS ResourceGroup, MeterName AS MeterName, ResourceLocation AS ResourceLocation, UnitPrice AS UnitPrice, PaygPrice AS PaygPrice, ROUND(UnitPrice * SUM(Quantity), 2) AS CostBeforeRI, ROUND( ( UnitPrice * SUM(Quantity) - SUM(CostInReportingCurrency) ), 2 ) AS SavingsFromRI, ROUND( ( (UnitPrice * SUM(Quantity)) - SUM(CostInReportingCurrency) ) / (UnitPrice * SUM(Quantity)) * 100, 2 ) AS Savings_Percentage FROM AZURE_COST_USAGE WHERE (PricingModel IN ('Reservation')) AND (MetricType LIKE '%Amortized%') AND (ConsumedService LIKE '%Microsoft.Compute%') GROUP BY timeInterval_Month, ResourceName, SubscriptionName, ResourceGroup, ResourceLocation, UnitPrice, PaygPrice, MeterName ORDER BY SUM_Quantity DESC ",
"needBackLinkingForTags": true,
"dataGranularity": "MONTHLY",
"timeRange": {
"last": 1,
"excludeCurrent": true
},
"limit": -1
}