Hi Alex,
There are obviously no $-number that I can share with you, but there is plenty of information available that can help you to formulate your KPI's
Here's a good cost analysis that is sheding some light on the cost distribution of a HANA Platform. http://www.sap.com/bin/sapcom/en_us/downloadasset.2014-04-apr-14-22.projected-cost-analysis-of-the-sap-hana-platform-cos…
A while ago I came across this SAP HANA Cookbook from August 4 2015 and one thing caught my attention was the recommendation made in section section 1.3.5.1
'You can reduce capital and operating hardware costs as well as other operating costs by shrinking the size of your database. Therefore it is strongly recommended to perform housekeeping and remove any unnecessarydata". https://proddps.hana.ondemand.com/dps/d/preview/0edf65cbbc3143b4b7216b1f5fd22040/1.0/en-US/loioccd52e1a22214551a7df8f027…)
Hope this helps,
//Peter