Bookmarks by Fosters is designed to help communities who are looking for a full-featured method to bookmark topics. You can browse a user's list of bookmarks, there are public vs private bookmarks, it's integrated into the Activity Stream, and there is batch editing of bookmarks with categorization. There's nothing else out there in the IPS Marketplace that fulfills this need, and you'll find a variety of ways in which Bookmarks is deeply integrated into the IPS software, which hopefully helps you understand the great value in this app for its price.
Not every app can meet everyone's budget. While I would also like every app on the Marketplace to be cheaper (or even better, to be free!), just because it's a cheap app for you does not mean it's a cheap app for the developer to code, maintain, support, and provide patches and bugs on a variety of different setups and configs.