Blood donations are a famous oft-cited case of where we might get less of something of we pay more for it.  Now it seems the problem is just with cash, not with payment; apparently we dislike an appearance of being paid, not payment itself:

We set up … a survey administered to 467 blood donors in an Italian town, and find that donors are not reluctant to receive compensation in general: A substantial share of respondents declared they would stop being donors if paid a small amount of cash, but we do not find such effects when a voucher of the same nominal value is offered instead. The aversion to direct cash payments is particularly marked among women and older respondents, while there are neither gender nor age differences in the response to the voucher.

More here.