Jaguar will
not be OS X 10.2. Read Apple's page carefully (or not so carefully). They go out of their way to never call it 10.2. It's Jaguar, the "next major upgrade" to OS X. My guess is that Jaguar is going to be called
OS X 10.5.
Historical precedent has been that single-dot upgrades (9.0 -> 9.1, 8.0 -> 8.1) have been either free or very low cost. Anything greater than .1 (8.1 -> 8.5) have been something closer to full price (I think I paid full price for 8.1 and 8.5).
I think we'll end up paying between $50 and $100 for the upgrade to 10.5. I may be wrong, but that's my guess. Apple may also be gauging public reaction over the next few months before setting final pricing (they should; they need to find the sweet spot where almost everyone will upgrade).
As for historical coverage, it can vary between three and six months, typically. New machines should come with upgrade coupons as well, although how those will be utilized is certainly not yet known.
This will not be a free download, and unlike 10.1, it won't be free at the local Apple store, either. Only time will tell how much money it will cost, and how many CDs it will come with -- the WWDC attendees received six of them (two for client, two for server, and I think two for dev tools?).
-rob.