Hello,
several months ago, my company came to me with nearly the same question. This is not so easy that you can reduce it to the cost reduction caused by saved time.
For HANA you need more RAM, more licence costs and dedicated admins which knows how to deal with it.
Of course, HANA is new, it's cool to have it, but it is not neccessary to use it when your system is e.g. containing a write-based solution.
You have to compare admin costs / hardware costs / licence costs, call it TCO if you want so...
at the other hand's side look if you need it or just want it. Than you have the solution.
My company made the decision for some lines and found afterwards that it wasn't good.
