How can I add extra information for each term reference?
There are two options:


Create own module based on taxonomy. Most flexible, but also most complicated. That's what I would do, if I had time and motivation to play.
Use Field collection - create a collection of term and percentage fields, and make this collection (not individual fields in it) multivalue. Warning: I know of no easy way to make sure your percentages adds up to 100%. I know how to do it easily in custom module.... see point 1.