When looking at ‘digital transformation’ within Government the measure is often a cost reduction in processing requests for services. Something which cost $10 per transaction moves to $1 and the $9 difference is measured as a cost saving over thousands of subsequent transactions.
This can be measured for digital services via website log files, and more easily with google analytics from multiple Government sites feeding into a dashboard. The UK’s GDS use this approach with great success.
Translating this to open data might be of benefit. For example, for each dataset you’d store in the metadata the FOI request charge, or some other relevant cost assumption. Having this data would allow both the platform owners and app developers to measure the assumed savings they are providing when people use the data.