Apparently 6 percent of internet users in the United States use Reddit in their spare time [1]. It is probably the same in Canada, Australia and the UK.
If we assume $0.25 in revenue per month per user and 20 million users, that's revenue of $60 million per year.
[1] https://www.statista.com/statistics/443332/reddit-monthly-vi...
They don't have to pay dang for that I'll be happy to oblige, this is a toxic comment because of that last bit, essentially you are just stirring the pot and baiting the moderators.
As for the value of HN to YC: it's a recruiting and marketing machine the likes of which you'll rarely find for any kind of money, I suspect that no matter what Dan and Scott are paid it is well worth it to YC.
https://www.reddit.com/r/announcements/comments/4mv578/affil...
Here is what Mr Huffman has to say on this topic:
Steve's tone here is pretty clearly tongue-in-cheek.