In reply to mike123:
I knew a guy who did this, oddly he was a student in Sheffield. I think he said he got £20 a pop which seems like a lot (this was c.1990). I never considered it as I wasn't comfortable with the responsibility for someone's existence, even if I might ever know about it.
I did do a medical trial of a diuretic once when I was seriously broke. It was through people I knew so I didn't feel I was going in blind but it didn't really feel right, though it paid better than anything else available to me at the time. There were guys there who were regulars and snapped up any and every test going. I think sperm donation might be preferable though I'm not sure how many shifts they'd let you put in.