If you already have vanilla HR... if you already have The Missing Link... the Director's Cut is not worth spending full price on.
But you can get it for $10 to $15 on Amazon right now. One of the best games I've ever played, with a whole slew of worthy content not found in the standard edition.
If you think it's an unreasonable price, don't buy it. You have a perfectly great game already. But seeing as you've never played the Director's Cut, you don't know what the new boss battles are like, you haven't explored the commentary, and unless I missed it, you haven't played the LONG-ASS Missing Link... you are in no position to claim it oughtn't be sold on its own. You couldn't possibly know.