It's also the wrong way to look at it. A better way is "what does it cost to apply" -- it's maybe 30 minutes to 10 hours of your time to fill in the application (it's more if you needed to do more thought about the business anyway). There is zero downside if you don't get in -- it doesn't prejudice YC against you at all in the future, and is confidential within YC -- and potentially high upside.
The ultimate test of "is it worth it" is to ask how many YC alums would recommend YC to someone else. As far as I can tell, that number is >80% (I can't think of anyone who wouldn't). I would recommend YC with only two reservations:
1) If you're doing a non-US startup (say, something local in China), and have exploding growth, no plans to expand to the world market, and unlimited capital, YC might be a distraction (although I doubt any company like that reads HN)
2) I think the biggest benefit to YC is to be able to launch during YC, or to have already launched. If you can't launch during YC, it might be worth doing YC later (when you're about to launch), or more likely, change your launch to something smaller that you can do sooner. There is probably 10x more value in YC if you launch before or during YC than after demo day. We made that particular mistake ourselves. OTOH, spending an extra year or two pre-YC building something no one wants is ALSO bad.