Bux or Aurora

There are two kinds of pay to click sites.There are so-called bux and aurora sites. Main difference between them is in amount they pay you for your click on ad.

Bux sites usually pays you about $0.01 (1 cent) or more per click and $0.005 - $0.01 per referral click. They are much popular then aurora sites because of the higher earnings. However those higher payouts cause many bux sites failures because they are usually higher then the amount they charge for advertising.
That's why Bux sites need other ways to generate income to sustain site. These include selling premium memberships, renting referrals, jackpots etc. 

Aurora sites pays you only a fragment of a cent ($0.0001-$0.001). That's almost nothing for many people, but those sites are much more sustainable than bux sites and last much longer.
With much lower payouts aurora sites can rely on advertising income to sustain site.
They also sell premium memberships and referrals, but they don't depend on them like bux sites does.
However, they are not for everyone because of a small amount they pays per click. Only if you can get many referrals you can think about joining those kind of sites.