Matrix table issue: drilling down to notes on a transaction (sample of what I want included)

Below is a generalized example of the type of table I'm dealing with. There are multiple notes associated with any given "win." I'd like both a summary of the this data as seen below and an ability to drill down to see the multiple notes for any given sale. What's the the best way to do this? I am guessing that building a matrix using average instead of sum will get me where I need to be. And then somehow I can simply sum the averages instead of having an overall average in the grand total. Maybe I just answered my own question... Is there a better or more elegant way to do this than using average instead of sum for the values?