Ad revenue, the first thing that comes in your head which is Google Adsense, yep it's no joke read.cash could do that, indeed all other sites tried that method.
The ad revenue required for maintaining read.cash's fund depends on the paid out (last week) in average in order to determine how many impressions required in order to generate a profit, not a loss. Also sponsorships have been recently there too but it's not enough, they a need a really big one, maybe Amazon or Red Bull anything that can make large sum of money. Adsense pays between 1 and 1.5 dollar per 1000 views, meaning they need about 8.48$ daily revenue which is about a minimum of 8,480 views per day. With such daily revenue it can supply the last week's payout.
Well, first of all AdSense is ugly. And I mean UGLY...
It takes a lot of space and gives very-very-very little in return.
I have another site that has about 2000 page views daily. At best it got $0.80 daily. That's peanuts, that's nothing. I took away AdSense after a month - it's not even worth the hassle, it's more like giving a charity to Google.
We don't know the exact amount of page views read.cash gets, but it gets 1,500-2,000 users daily, so maybe 2,000-3,000 pageviews. So, we'll get maybe $20 per month (minus taxes). It's close to nothing :)
Don't worry too much about the fund - we're only testing things up, it should get bigger.