I agree: ALWAN Cmyk-Optimizer is really a great tool for that!
(The price ist not really low, but just have a look to your yearly amount of ink-bills and math with about 10-15% up to maybe even 30% less after using ALWAN is possible whith an adjusted GCR, but it depends from your Jobs/Portfolio/restrictions/settings for handling the files. But we use it mainly to lower the TAC for minimizing the risk of a glued stock, what is also to "count".
Might be ALWAN will let you work with a Demo for a while, then you can check the saving via the software, it is build in there a statistic window to controll the costs.)