Assuming the current business model (free, with no ads), HN's valuation as a stand-alone business should be negative, since it will lose money (hosting, moderation and maintenance costs) as long as it keeps operating.

If there was a commercial competitor to HN, it would have to be significantly better than HN to attract users away from HN. HN's biggest attraction is a relatively small number of very knowledgeable and interesting users, and that would be very difficult for any commercial competitor to duplicate. And without such a user base, an HN competitor would also probably have a pretty low valuation.