i dont think 100$ is that absurd, the account probably took a good few years to gain that much Karma.also the large post base makes it far more versatile.
i think a good starting bid would 50$. anything below that just keep the account
Reddit karma has no value, you say that it takes time to gain that much karma but in reality most of the karma on Reddit is gained by reposting old material to default subreddits. Karma has nothing to do with account age, a few good posts could earn an account karma up to the ten thousands. Therefore karma isn't that of an important factor when it comes to Reddit accounts, it comes behind account age and comment volume & quality. And yes, 100$ for a two year old account is absurd.