> It's at least little bit fishy that you can get a $500 million deduction for giving away stock that is never worth $500 million to anyone (since giving it away tanked the sale price, and selling it would presumably drive it down even farther).

I'm not entirely sure you can, based on my reading of 26 CFR 1.170A-1(c)(3) [1]:

> (3) If a donor makes a charitable contribution of property, such as stock in trade, at a time when he could not reasonably have been expected to realize its usual selling price, the value of the gift is not the usual selling price but is the amount for which the quantity of property contributed would have been sold by the donor at the time of the contribution.

[1] https://www.law.cornell.edu/cfr/text/26/1.170A-1

Excellent point. I responded to your other comment on this thread.