I have several years experience in this field. Rent Referral comes from most of PTC sites is irregular to click ads, which can change your calculation greatly. You can buy 3/5 rent referrals in exchange of 0.6USD, in order to earn 0.6 USD, you have to click more than 120 ads or you have to wait more than 15 days ( Though it totally depends on the condition of PTC sites). Your work will in vain if rented referrals do not work regularly. So, it will be good to collect money in stead of buying rent referrals.
Rented referrals coming from the following site is working properly ( need not to be my referral, you can join by yourself to compare)