I've done it. $220 for the first four donations, IIRC. Then $65 a week for 2 donations...$25 the first, $40 the second. I think the price has dropped since then.
After the first session (which takes the longest since they need to run a bunch of tests) it's usually about an hour-long process, all told. They stick you to test for plasma quality (I forget exactly what they looked for), then you go to the back room and watch trashy TV while they put a clean needle in you and let the centrifuge machine take over. Everything that contacts blood is new every time...this stuff is being used for big pharmaceutical companies, even if they don't care about the donors they can't afford to let disease pass to the end user.
IMHO it's worth it for some quick, easy cash for a broke college-age person, but after 8-10 sessions you probably want to stop. You can donate up to twice a week, but it starts getting more painful as it doesn't completely heal in 3-4 days. I didn't want to get to the point where I was putting my veins at risk.