Nope, the reason you can't find the rule is because it doesn't exist.
Class skills require 1 skill point to add a rank.
Non-class skills require 1 skill point to add a rank.
Same as Cheapy said, the only difference between class skills and non-class skills is the 3 bonus ranks you receive at the begining.
The rule you're thinking of is from 3/3.5 D&D. It was not carried over into PFRPG, and was compensated by eliminating the x4 skill ranks at first level.
From the SRD:
Each level, your character gains a number of skill ranks dependent upon your class plus your Intelligence modifier. Investing a rank in a skill represents a measure of training in that skill. You can never have more ranks in a skill than your total number of Hit Dice. In addition, each class has a number of favored skills, called class skills. It is easier for your character to become more proficient in these skills, as they represent part of his professional training and constant practice. You gain a +3 bonus on all class skills that you put ranks into. If you have more than one class and both grant you a class skill bonus, these bonuses do not stack.
It's a much more elegant and simplified system, IMO.