I feel like agreeing with nick. There is no point in investing in any ptc site for referral buying.
Referrals are like lotery. You never know who is going to remain active for how long. Most of the times the referrals leave abruptly and you do not gain anything.
On top of everything else many ptc sites are proving to be scams. They are not paying regularly. So I think that it would be better option to have referral exchange instead of buying the referrals.