well, I blieve while people buy the traffic/revenue name, they made an assumption first, the stat is there, see 20$/m, and assume it will keep stable...(some people want give a 20% downgrade while continue the payback caculation...in this way, people assume the domain will avarage earn 16$/m in future), and you need valid this assumption, some factor may impact this assumption like:
1. that is an old domain, some visitor continue visit it but after 3 month, no one will revisit it again..
2. the seller AD it and get the un-nature click
3. some interesting news/event bring more click just recently, while after this event, the visitor will decrease as well..
..etc
after valide/establish the assumption, then u need conside payback, the caculation is if U pay 720$ for it, the payback is 3 years in this case(still using 20$/m as the base)....
actually, if u have no other good investment channel, 3 years payback is good enough, not consider the domain itself have other value, may be some end user want buy it at 1000$...u have a lifetime to wait this kind of opportunity.....
so, for me, the strategy is
1. any domain if can earn the reg/renew fee itself is worth to reg/renew, u lose nothing while earn an opportunity to sell it in future...
2. if this domain can earn 20$/m. then 3 year payback if good enough to buy it, while the conside point is:
a. u need valid that assumption, if it really can generate the 20k/m or 16$/m
b. u lost the oppotunity invest the 720$ in other area..(opportunity cost I remember is the term accountant use) (eg: some other invest area can provide a 2 years payback)
c. u still earn an lifetime opportunity that domain may can sell at a great price in future...