Would the people in the HN community stick around if HN started showing ads and selling their data? They could easily move to a similar site, such as lobste.rs.
Would people pay money to access HN? Much of the value of HN comes from the users who spend time sifting through a lot of junk articles and comments to upvote the tiny fraction of interesting ones. Are you going to ask users who create the site's content to pay for it?
If you can't monetize the site without driving its users away, then it's worth $0.
There is some way to put a number on that, but few organizations could get the value Y Co does.
For a start Wikipedia says the top Y Co companies are worth $155 billion, Y Co gets 7%, so Y Co has made $10B or so, if HN is responsible for 1% of that, that is $100m.
Another method is to use the $1MM/engineer rule. There are 2official mods and 1 official developer (I'm not sure if his work is not shared with another YC projects), and there is an undisclosed amount of semi-mods, and there is some support crew that is probably shared with YC. So I guess $5MM or $10MM.
Monetization is a problem. Reddit was never a monetization success. StackOverflow had better luck with that.