I am wondering if anyone who uses commission percentages to track deal revenues can tell me how they set up their products and track revenues?

It seems cumbersome to figure out how to determine revenue on a piece of business.  You can only choose (or at least I only know how to choose) Price, Quantity, Tax%.  Could we look at adding a Commission Percentage field to products or a custom field that can do the calculation?

If this already exists, could someone tell me where?