Another great little app that I own, although the current version is feature limited compared to the competition. I'm confident that v2 will bring it right back to the top of the best twitter apps though.
As far as pricing for upgrades to v2, there was some discussion a while ago that v2 will be a free upgrade. In this Tweetie2 for iPhone TechCrunch article the author is reported as saying so.
"Brichter has also revealed that he is working on Tweetie 2 for Mac, and that it should be available shortly. He doesn’t give away too many details, but there are features such as syncing between the iPhone and Mac version. That will be a free upgrade if you already have a license for Tweetie for the Mac."
So looks like it'll be good news, but don't blame MacHeist if it's not, Loot is often version upgrade limited