There isn't a (life saving) organ you can donate that wouldn't put you at least somewhat at risk - kidney, liver segment, etc. Organ donors have to undergo a ton of evaluation to make sure they're donating for the right reasons and will be capable of dealing with the possible outcomes. Incentivising that with money would be a bad idea because people who shouldn't donate would offer themselves up.

You CAN, however, make bank on hair, plasma, eggs, bone marrow, and plenty more.