Ironically, answering it requires looking carefully at what my physics profs often referred to as "First Principles." In today's world, networks answer to the CIO like every other IT cost center. And defining network performance starts with figuring out what the business needs.
So the leading question should be: What is the business case?
If you miss answering this one, you can easily come up with seemingly substantial responses that are built on implicit assumptions. And in the end, they may be as meaningless as "30% more capacity with new ultra-fast transfer protocols and packets up to 8 times bigger." Sounds like bad marketing.
Once you have the business case clearly defined, then requirements can be extracted, metrics selected and evaluations can be made. Measuring the "value" of a "network solution" is just a matter of meeting or exceeding the minimum requirements within the budget provided.
So if you start looking at your "network solution" in this light, I suggest you review some of the "first principles" as they are laid out in SearchNetworking's Guide to network performance management and in particular, the background on the network life cycle. After that, there are a range of metrics and methodologies for assessing these described, appropriate for a range of different business needs.