http://registerfly.com
Don't pay attention to whatever price it gives before you sign up. When you make an account, they send you promo codes at least once a week and usually more. I just transferred a bunch of domains last night for $6 each.
There is a place on the main page of your account that says "promotional codes" or something like that. When you enter the codes there it will tell you how long the deal is good for and update your main account page to show what your current costs are for each service.
The codes are on the promo emails they send. A lot of times it will say it's only good for 24 hours or 3 days, etc. Even if it's past the deadline, go ahead and put the code in on your main account page. It always takes it on mine, even if it's expired. Or just come here and ask who has some registerfly promo codes. They're always generic stuff like FLYPROMO6 so I'm sure they'll work for everyone. If you need some for right now, I can post my latest ones.