Lets pick on the one I think is easy here - a Taliban source. The US spent 20 years in Afghanistan, wasted trillions, murdered almost a million. Opportunity costs even bigger than the needless waste of course.

How much is that Taliban source worth vs. greater transparency that could have ended the war earlier? The biggest problem was publicising which interest groups in the US government were responsible for prolonging the inevitable. Just keeping all meeting minutes on a website unredacted would have been a lot more valuable than having a source.

The tricky one is the Saudis. How much is a Saudi source worth vs. full transparency of voters into the US-Saudi relationship? The issue here is ... we can't debate that, the necessary knowledge is secret. But since large organisations are generally dysfunctional, and there is no reason to believe that the Saudi source is more valuable than more transparency into what is actually going on in the Middle East.

The issue to me is that secrecy makes democratic institutions ungovernable - they can't be assessed without full information and therefore voters can't even attempt to make rational decisions. Full transparency is probably more valuable than the net influence of secret sources [0]. The value of long-term secret sources is highly questionable. If there is a source or confidant in some foreign organisation you want to protect, give them a passport and set them up in Texas. Problem solved.