Without intending to derail this thread, I'm surprised that a cosplay group can afford a pro to follow them around.
I would be astonished if they don't balk when you name a price that's fair to you. They might be expecting a hundred bucks a day, when your usual rate is a hundred bucks an hour.
You might instead suggest that instead of following them around a con for four days, you instead set up a few hours for posed shots, with a couple of costume changes per person. Charge a flat rate for a four-hour session, make sure it includes fair pay for postprocessing, and includes a fair price for image rights if you're giving them a CD.
They'll probably have a couple of members who will know how to do posing, and/or they'll have poses pre-determined for the characters as whom they are cosplaying. It's likely that you won't have to spend a ton of time doing posing - you'll be able to blow through them pretty quickly, and get more/better results than a ton of candids at the convention. Maybe have them send you some character background in advance of the shoot, so you can be prepared with lighting/props/background/location that suits the character's style.
Having said that, anime and comic conventions are a ton of fun for getting photos of people dress in all sorts of costumes, including some that are extremely elaborate and well-done. And, most of the costumed attendees are very eager to pose for a camera, especially if you look like a pro. If you have the time and the inclination, don't treat it as a professional work, instead treat it as a fun time and an opportunity to get some cool shots of some fantastic costumes.
Some more reading:
http://www.cosplay.com/forumdisplay.php?f=173