I would expect Tiger to be $129/£99, just like Jaguar and Panther.
Mac OS X 10.1 (Puma) was free (or $20

) because Mac OS X 10.0 (Cheetah) was incomplete in many obvious areas. Of course it was only free to 10.0 users, which is fair.
I doubt Tiger will be more than $129/£99 because I can't see it taking longer to develop than Panther. I bet it will be out at around October again, only a year after Panther. I say this because announcing its features so early at WWDC begins to hurt sales of Panther as people then expect it to be "around the corner", like they did last year. So they need to then get it out as soon as possible.
It would be nice to see some incentives for .Mac users though. If I could get Tiger for half price as a .Mac user, I'd sign up, and this could be a good thing for Apple.
Regarding the numbering system. Mac OS X is Macintosh Operating System version 10. They could have called it Mac OS 10 but they used the roman numerals to distinguish it from the old classic Mac OS. This also explains why Mac OS 9 was rushed through and didn't last that long: because 10 is a nice round number to start the next generation.
I expect OS X to continue for many years, maybe even outdoing System 7. We'll see 10.4, 10.5, 10.6... - there's no reason why we can't have Mac OS X 10.10 or 10.11. That would still be OS 10, not 11. So, unlike what JFreak points out, Panther is 10.3.4 not version 3.4 of a new system called X. Personally I find the numbering system much better than the mess that Windows has got itself into.
Of course, expect the familiar outcries when we have to shell out more money to upgrade later this year and, of course, iLife '05 in January.