The question I have here is this even worth it? You'll constantly lose followers you've been working on getting for a possible small exchange of money. In the end, the traffic that twitter drives to your sites from your own personal tweets can be much more valuable if you have a decent size following crowd. However, I also believe the only way this would make any type of money is if you had a lot of followers anyways. Sounds like a good way to lose a good marketing tool.
I'm new here, but I've noticed that a very trusted site to do pay-to-tweet campaings is sponsoredtweets.com. You sign up (and yes you do have to give your screenname and password), they tell you how much you should sell your tweets for, and follow the rest of the directions to complete registration. This is the site that the cemebrities go through, so you dont have to worry much about your login info being compromised. Sorry for the people that already know about this, but just thought this is a SAFER way to go.