Not 'most likely': Apple has already said so publicly. 129$. And whether you see that as an upgrade or retail price doesn't really matter, as everyone who uses a Mac that can run Tiger already _has_ a license for Mac OS.
About using Tiger as your primary OS: I wouldn't just yet. Wait for the next build or so. I've been doing it (betas as primary OS) for years. With a good backup plan, that's going to be okay most of the time, but as long as too many incompatibilities throw you back to Panther, it's not worth it. I hear the 8A2xx builds are quite good, and I'm certainly going to try the 8Bxx builds as my primary OS again... Mostly because Spotlight only makes sense when actually using it with real data...