It costs $79.95 for a SageTV license which includes unlimited guide data piped directly to the client looking forward 2 weeks for North America and some different provisions for the rest of the world. That's it. There's no OS to buy if you run Linux, just like with Myth. $20 for a software-only placeshifter license which you'd run on another "client" computer (from anywhere on the net like SlingBox).

I think compared to Myth's price of "free" it's a wash because of the much lower price of an extender (which doesn't require a Placeshifter or any other license).