i used to donate plasma when i was dirt poor. id keep a few dollars for cigarettes and give the rest to my roommate for bills.
it usually only took 60 to 90 minutes. you'd be hooked up to the machine which would pump out the blood, run it through a centrifuge, and then pump the blood, minus plasma, back into you. theres not much that can compare to the feeling of cold blood being pumped back into you and it begins to re-circulate through you body, brrrr!