This API is basically it is the same one described here. Where all properties are described as well. Basically costWithNoReservedInstances and totalCostWithReservedInstances values are in units and the type of unit is in the meter you find for meter ID with 7fe518bd-edbc-5c4a-a364-0bc7d5139819" in this case. For VM compute this is I think always one hour. So basically, these are the hours the VM. Let's say one hour costs 0.21$ you will multiple 6.287733968000001 x 0.21 and 2.3072564687974566 x 0.21. That way you will get the cost for the compute that you will pay without RI and with RI instance. If you look at lookBackPeriod property you will see that this was the usage that was generated for the last 30 days, netSavings is how much units would you have saved and it is the difference between costWithNoReservedInstances and totalCostWithReservedInstances. term is what reserved instance period you can buy and if you multiple that quantity by 36 months (as the data is for last 30 days - one month) you will get how much you will pay for compute without RI and with RI.
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