If we accept that selling organs is prohibited and unethical then we should also accept the same for selling plasma.
Do we agree that selling organs is a bad idea though?
* The Sale of Human Organs (Stanford Encyclopedia of Philosophy) || https://plato.stanford.edu/entries/organs-sale/
* Why Legalizing Organ Sales Would Help to Save Lives, End Violence - The Atlantic || https://www.theatlantic.com/health/archive/2011/11/why-legal...
* Cash for Kidneys: The Case for a Market for Organs - WSJ || http://www.wsj.com/articles/SB100014240527023041494045793225...
* Most people are okay with 'selling' organs — and it might be wise - Business Insider || http://www.businessinsider.com/can-you-sell-organs-2016-9
And... it's not like the rich aren't already buying organs... why not legalize, make transparent, and regulate the process for all?
* Liver Transplant for Steven Jobs of Apple Raises Questions - The New York Times || http://www.nytimes.com/2009/06/23/business/23liver.html
* Steve Jobs' liver transplant: Did he game the system? || http://www.slate.com/articles/health_and_science/human_natur...
Seriously, egg donation seems to affect your health more than plasma donation. According to wikipedia:
Birth control pills are administered during the first few weeks of the egg donation process to synchronize the donor's cycle with the recipient's, followed by a series of injections which halt the normal functioning of the donor's ovaries. These injections may be self-administered on a daily basis for a period of one to three weeks. Next, follicle-stimulating hormones (FSH) are given to the donor to stimulate egg production and increases the number of mature eggs produced by the ovaries. Throughout the cycle the donor is monitored often by a physician using blood tests and ultrasound exams to determine the donor's reaction to the hormones and the progress of follicle growth.
Once the doctor decides the follicles are mature, he/she will establish the date and time for the egg retrieval procedure. Approximately 36 hours before retrieval, the donor must administer one last injection of HCG hormone to ensure that her eggs are ready to be harvested. The egg retrieval itself is a minimally invasive surgical procedure lasting 20–30 minutes, performed under sedation (but sometimes without any). A small ultrasound-guided needle is inserted through the vagina to aspirate the follicles in both ovaries, which extracts the eggs. After resting in a recovery room for an hour or two, the donor is released. Most donors resume regular activities by the next day.
Yes, for a healthy individual, the act of donating plasma is about as risk free as you can get. Perhaps sperm donation is the only less risky live tissue donation. The reason we pay for it is that, if we didn't, we wouldn't have enough, and people would die. That's not speculative - countries which don't pay for plasma donation end up buying it from the US.
Donating eggs can have really serious complications even for healthy individuals - its not a trivial procedure, and even when performed correctly, it carries a small risk of infertility.
That's not to say you shouldn't do it, but there's another reason that we shouldn't expect women to do it for free - that actually becomes a lot more exploitative than providing up-front compensation and a full explanation of the risks, which we do.