In our society, this common currency of expected utilons is called "money". It is the measure of how much society cares about something.

What is this 'society' of which you speak?

And why should we expect a common currency of utilions? Money can buy you lots of things, but not anything. Plenty of people give up lucrative careers for more satisfying ones. That doesn't negate your overall argument, but I think you're wrong on that particular point.

My view is that charity has less to do with actually helping the needy than with signaling compassion to others. If that's right, the relevant metric of charitableness could well be something like resources expended rather than benefit produced. Additionally, helping at the soup kitchen may be more visible and salient to others than giving money. A lot of people would be reluctant to actively inform others that they'd given $100 to helping the needy, but feel comfortable trumpeting their volunteering to all and sundry. If someone asks you what you've been doing, it's much easier to say 'helping at a soup kitchen' than 'working an hour and donating the proceeds to charity'.