Well it depends on what the dealer charges for labor, and what they charge for the part.
Usually the dealers' parts are going to be a little more expensive than what you'll find online.
Could also be an issue of the dealer estimating the cost on the high side, as most of them do. That way, if for some reason they ran into something else that needed to be replaced, they wouldn't have to call you with more bad news.
If the magnet came off, was the stator damaged? Haven't heard of a magnet coming off without the flywheel being struck hard enough to jar one loose. Even then, it'll stick to the flywheel....most of the time. If it came loose while the engine was running, the stator is likely damaged and that combined with the flywheel and labor, would easily add up to $700.