No (fungible) US dollars?

Hmmm, I suppose all I would need is a botnet and a bitcoin address to change this. That's somewhat beside the point, though.

The economic language here ("exchange rate", "paying", etc.) is not meant in a serious sense: obviously the donors were not making a donation for the sake of gathering karma. But yet, they did signal, and they did (in a non-serious sense) purchase that signal.

Donating to charity is a less efficient way to gain lesswrong karma than googling for rationality quotes even in terms of time taken to make a donation, neglecting the cost of a donation itself.

Of course it is. As I wrote,

Naturally, there's little point to this analysis. If anyone is trying to maximize net karma by donating to SIAI, something is probably wrong with their priorities.