Dan Ariely dedicates an entire chapter in Predictably Irrational on when payment is and isn't appropriate and the chaos that happens when you screw up. It's worth a read. The short version is this: let's say you pay a token amount. All of a sudden, the people for whom that token is inconsequential take it as an insult. "Sitting there for half an hour literally giving you blood is only worth $10 to you? Piss off!" So let's say you instead give a healthy percentage: at $100 per unit, all of a sudden you're filling your halls with addicts and derelicts that are doing everything they can to fake a blood test which drives away the people you want to donate. Meanwhile, you're taking a "favor" transaction and turning it into a "business" transaction which sets the value for both parties at whatever you arrive at: "We're paying you $20, we don't have to treat you nicely." "Screw this, it's not worth putting up with this bullshit for $20." If you instead leave it as an intangible, both parties get to set their price in "warm fuzzies" - "I helped save a life and got all the juice and cookies I wanted." "The world is full of generous people who give of their own bodies for free."

The California Bar Association saw their volunteer enrollment jump a thousand percent when they dropped their recommended fees from "25% of your hourly rate" to "nothing." As it turns out, people who get a free lawyer tend to treat them with respect, while people who get a lawyer for 75% off tend to treat them as if they got them full price. Meanwhile, lawyers who work for free feel like they're doing good, while lawyers who work for 75% off feel like they're getting ripped off.