How much I can earn?
As explained earlier you will be paid between $0.0001 to $0.1 for every watched ad. So if you want to earn more from PTC sites you have to upgrade your membership or else by renting or referring referrals to work and you will get some commission till your referral lifetime. 
Referrals are those people like you who joined these PTC sites directly through your referral link. 
PTC sites provide referral link for every registered member who can use that link to refer others to register. 
They are called Direct Referrals. If you want to find the way of getting PTC referrals do visit here to know how? 
If you could not able to bring direct referrals, then you choose to rent them. 
PTC sites itself offering this service and you have to pay a rental fee referral per month. You can either bring direct referrals or rent referrals to increase your earnings in PTC sites. 
A simple calculation: Your click earnings: 5 ads x $.01 = $ 0.05/day Your referral click earnings: 50 x 5 ads x $.005 = $1.25/day
Your total earnings per day per site = $ 1.30. Your monthly earnings per site = 30 x $ 1.30 = $39 If you join just 10 best paying PTC sites, your potential earnings is: 10 x 39 = $390/ month