Drupal rules discount matrix - look up by taxonomy term
If you want to use rules (and with commerce I think it makes sense), and you are comfortable with PHP, I would recommend writing your own module that defines the custom conditions and actions that you need to look up and apply discounts. 

In my experience, the rules interface starts to break down quickly when use cases become even moderately complex. You can spend a lot of frustrating time fiddling with the interface, trying to get it to do what you want.

By writing your own conditions and actions, you keep the complex logic in code, where you can easily debug, access, and compare any information you might need.

In addition, your custom actions and conditions will be nicely exposed in the rules interface, and keep the high-level logic of your commerce solution tidy in one place.

Docs are here: https://drupal.org/node/298486. Drupal commerce itself has many good examples of custom actions and conditions to use as reference.