How to extend Pathfinder point buy to scores below 7?

In Pathfinder point buy, ability scores of lower than 7 (before adjustments from race) may not be bought with point buy.

What would be a balanced price for such low ability scores?

I am looking for personal experience from people who have done such an extension, or for a reasonably accepted view in the character optimization community.

Motivation for the question: How likely is to roll elite array or better with 3d6 in order?

In particular, I am not worried about players creating horrible malformed monsters with only threes and eighteens as their ability scores.

The question Is it allowed to buy ability scores lower than 7? addresses whether buying low ability scores is allowed (no), which is not the question here. The answer also argues that allowing buying such scores is a bad idea. My use case is very different – I want to have a method of measuring how good or bad ability scores are, and allowing anyone to buy them is not an issue here.