If a board has exercised prudence in its decisions over difficult issues it is generally thought to have met the common standard for fulfilling its duty of care.
And what is prudence you might ask? Prudence is how the "majority" of people facing a similar situation or decision would respond. Prudence is not as much scientific as it is experiential -- you kind-of know it when you see it and more importantly, recognize it when it has not been used. On the other hand, there is a formal definition: "the use of good reason" often including the characteristic of "caution".
All that said, examples often help to demonstrate what is not "prudent" and the General Services Administration provided a live one last week! Specifically, two GSA deputies spent $823,000 on a conference in Las Vegas. Some of the must haves: a clown, mind reader & comedian, boxed commemorative coin sets, novelty yearbooks & souvenir canteens, and a $75,000 "team building" exercise. The head of the GSA and two top officials have since fallen victim due to this questionable use of government funds.
The good news: it's not that hard to find stories about what is not prudent decision-making. As a simple rule, if a board has doubt as to what is prudent, they might ask the question: what would (name a person generally respected) do in a similar situation.