You will get many different opinions on the subject, but there isn't only one set of numbers that work. There are too many variables to set this in stone. Your ballpark can make a major difference, for one. Also, the cap size definitely needs to be taken into consideration.
In an $80M league, I go anywhere from $28/IP up to $33/IP and more. My best advice to to hunt for bargains at lower price tags. There are some excellent bargains at $26/IP and even lower.
Good luck!