The last version of Mac OS X that was released was and is still $29, although Apple strongly suggested that anyone not running the most recent version prior to that should buy the $169 Mac Box Set.
Every previous version, except 10.1, sold for $129.
Chances are, Lion will be somewhere in between. It could be as cheap as $29, or as much as $129, and of course, it could be any other price Apple decides, but I suspect it will be somewhere in that range.
Apple has not typically offered upgrades for cheap until after they announce the release date, at least. That is to say, if you bought a new Mac already, you won't likely be eligible for a cheap upgrade, though there will likely be a 2-week grace period, simply because you could return the computer and rebuy, without any restocking fees now. Don't count on that, however, as they may not start the deal for a little while, depending on the release date and if they actually announce it.
jW