If you're looking for iChat AV, then it's $29.95 for Jaguar users. The one released before for Jaguar was a public beta that has now expired.
If you want the version that came with the OS, then I'm sure there's a way to install it from the Jaguar CD if you still have it. I believe there's a way to install it without completely restoring the system, but I'm not sure how.