There is a story in the news about Nikki Haley (now former US ambassador to the UN) taking trips in a friend's (and political supporter's) private jet.
According to the article, Haley declared these trips, but rather than declaring the true market value of a private plane ride from New York to South Carolina (~$24,000), she declared the value to be that of a first class flight on a commercial airline (~$3200).
This actually makes perfect sense to me. She didn't receive $24,000 of value. She can't sell the flight she got on eBay. All she got was a free trip, which saved her about $3200. So it seems reasonable to argue that the value of what she got was indeed about $3200.
So if we start valuing things in the way this watchdog group is demanding, then what happens if a politician meets Warren Buffet? The market value for eating lunch with Warren Buffet is $650,000. If Buffet would have invited Halley out for lunch, would Halley have to declare a $650,000 gift?