I am very, very wary of wading into anything approaching a debate with you, given my respect for you. But I feel that this comment assumes an unrealistic picture of how time/money tradeoffs work in most people's lives. Most of us do not have direct ways we can translate a couple of spare minutes into the corresponding amount of money, and even if we did, we aren't perfect utilitarians who always make as much money as we possibly can and then donate every remaining penny to the most efficient possible charity. If anyone is that kind of person, they should indeed act as you suggest.

However, most people have some inflexibility in terms of how much of their spare time they can trade money for, and how much of that money they feel prepared to give away. If you can spare 3 minutes to swab your cheek that you would not otherwise spend to earn 3 minutes' worth of money and send it to the Against Malaria Foundation, then you should probably consider that "free" time. Then the calculus shifts over to the odds that your donation, should you be asked to give it, would save someone's life. You estimated a generous 25% ($500) -- I don't know, but I'll go with that. In that case, the time would probably be worthwhile.

Let me emphasize that I understand I am not assuming perfect rationality or perfect utilitarianism, but rather how these kinds of tradeoffs are likely to play out in ordinary people's lives.