Just 25 € per User, supports offline modification and re-sync. Unfortunately currently only available for US and UK operating system versions!
€12000 € for 50 User (240 € / User), does not just sync: the user has the same Sharepoint Browser-based user interface when he's offline like when he's online. e.g. full-text search.
Groove Virtual Office does support Sharepoint Synchronization and starts at 179$ per Client.