What should be the ideal ratio in keyword research?

[BackUP]

For example: If the Avg. KC - difficulty of ranking is 30 and below, then it is considered easy. With a monthly search volume of 170 searches, meaning low competition, how much should the Rank Value be for each click to be effective in terms of bid? I am not sure about this, it seems to be related to the advertiser, but I am confused about what the average should be.