Modify node fields per user

Hello, I need help.

Situation: Customers can have any discounts applied, depending on the price discount field_price has to be updated (solved with rules), but then I have the field_old_price that have the defaults values in database but depending discount applied have to show other data.

Problem: How I can do to get these valors changing on the fly depending on the user that is on the web?

Proposed solution: I wanted to make a change when a node load products (node-type=product_display), edit these fields on the fly (without changing the value in the database) and display it using, for example, the hook_node_view, but I can not access form fields, i think that not a good solution.

Any suggestions or help? Many thanks!