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.
This topic is not really related to the things normally discussed here, but I think it's really important, and it might interest Less Wrongers, especially since many of us are interested in ethics and utility calculations that are essentially cost-benefit analyses. Bone marrow donation in the United States is managed by the National Marrow Donor Program. Because typing donors for matching purposes can be costly, they often require people signing up to donate to pay a registration fee, which probably prevents a lot of people from signing up. These costs are being covered until the end of the month by a corporate sponsor, which means that right now, all you need to do if you live in the US is go to http://marrow.org/Join/Join_Now/Join_Now.aspx and fill out a simple questionnaire. You will be sent a kit to collect a cheek swab, and then you will be entered into the donor database. Doing this does not require you to donate if a match comes up.
The reason I think this might interest Less Wrongers is that this is a really cheap way to improve the world. According to their website, about 1 in 500 potential donors are actually asked to donate, so registering doesn't actually make it all that likely that you will be asked to do anything more. If you ARE a match for someone who needs a donation, the cost to you is at most the temporary pain of marrow extraction (many donors are asked only for blood cells), whereas the other person’s chance to live is much improved. This looks like a huge net positive.
Unfortunately I only found out about this a few days ago, and it only occurred to me today that this might be a forum of people who would respond to the argument "you can make the world better at little cost to yourself." However, I ask that you go to the website and spend a few minutes signing up. This is like buying a 1 in 500 lottery ticket that SAVES SOMEONE’S LIFE. If the Singularity hits and an FAI can generate perfectly matched marrow for anyone who needs it from totipotency-induced cells, that will be wonderful, but this is a chance to make sure one more person gets there.