If the sites are on a root domain, then you can put ADWords on the site and get paid, but not much. Typically a thousand impressions is a dollar. Depending on how long the URL stays on the frontpage, you could be looking at 20,000 - 50,000 impressions which roughly translates as USD 20-50.

You get paid even more when ADs are clicked on.

To be honest this is a rather dated way to monetize a site now with the sudden surge of visitors using ADBlockers, and you might want to look into other ways to monetize, such as

- Affiliate links

- Premium/paywalled articles / content

- Donation buttons, using PayPal / Bitcoin/Litecoin

Also keep in mind that since getting frontpage on HN is so rare, then it can't be a sustainable source of income