Note: Be the Match is only available for people living in the United States.
Be the Match is a sign-up list for people who would be willing to donate life-saving bone marrow to a compatible patient. The way it works is that once you sign up, you are mailed a swab kit which you rub on the inside of your cheek and send back. Be the Match keeps your data on file, and whenever somebody needs a bone marrow transplant, they scan through their database to find compatible donors. If it's you, you get a call.
The sign-up process takes maybe an hour including the time spent mailing the package. Be the Match estimates that the entire donation process, if it ever occurs, takes 20-30 hours spread out over 4-6 weeks, including phone calls, medical appointments, and the donation itself.
Let's say the average person reading this post can make $25-$50 an hour. That means they are spending the dollar equivalent of $500-$1500 to save one life, in the event that they are called upon to donate. I'm relatively new here, but my understanding is that that's reasonably competitive. It also takes the form of hours instead of dollars, which might make it more viable for some folks (and maybe less viable for others).
You can sign up here. Happy donating!
Note: If you are 45 or older, then there is a $100 registration fee (otherwise, it's free).
Note #2: Down below, user HaukeHillebrandt has pointed out a reason why this might not be as great as it sounds, and user MichaelStJules has pointed out a similar reason (see third bullet point of the linked comment).
Worth noting (some of these are also mentioned in my other comment, with sources there):
1. If you're male and 16-30 (a significant proportion of EAs), the chance of being asked to donate is more like 1 in 200.
2. I'd say the time cost of registering is closer to 15 minutes than an hour. Personally I filled in the form at work during my lunch break, and spent a few minutes doing the cheek swab at home.
3. It's not clear that not doing a transplant would be cheaper for healthcare systems than doing the transplant. In general, complications for conditions which would require a transplant can be expensive to treat, and healthcare systems like the NHS would treat them. See the linked Guardian article in my other comment which suggests that transplants could actually be saving the NHS money vs treating the symptoms. Also see the other link that suggests UK transplants from UK donors are much cheaper than UK transplants from foreign donors (assuming these transplants would be happening anyway).
4. I do think health concerns to donors need to be taken into consideration. Most people (90%) can donate in a way similar to blood donation, but 10% undergo general anesthesia. This isn't risk-free, though generally the criteria which filter for suitable donors upon registering exclude people who would be at high risk when undergoing general anesthesia.