It is my impression that Spanning Sync is comprised of four elements: Your Mac, the client software, Google and an intermediary database server owned by
spanningsync.com. This last server would be the reason for the subscription fee if I'm right. The client software syncs changes between your mac and their server. Their server syncs changes between Google and their server. Doing it this way allows for their server to handle formatting changes and any changes made by Google without having you update the client.
I think that the $25 price is very fair for a system that let's me sync my contacts and calendars between my Mac, iPad, iPhone, Android phone and Google. Sadly though, with all of the little gotchas between Apple, iCloud and Google, I was unable to get thing to work reliably without having to jump through a lot of hoops. I've decided to can the whole idea and just use the Apple native solution and ignore Google/Android syncing with the Apple devices.