How to extend Pathfinder point buy to scores below 7?

Question

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.

Answer 1

The formula is that you have to pay for an ability score the new modifier. So going from 7 to 6 gives you a price of -2, 6 to 5 and 5 to 4 each give you -3, and so on. Dropping an ability score to 3 would give you 16 points.

Score | Mod | Cost  
   10 |   0 |    0
    9 |  -1 |    1
    8 |  -1 |    2
    7 |  -2 |    4
    6 |  -2 |    6
    5 |  -3 |    9
    4 |  -3 |   12
    3 |  -4 |   16

It's uncertain whether those values would be balanced ; while they follow the pattern of ability score costs, nothing says that this pattern won't fall apart outside of the 7-18 range. Furthermore, in any case, this should never be an option available to players as discussed in the answer to this question.

Answer 2

If I were to houserule it, I would say that going below 7 grants you one extra point per reduction(or possibly less!). The reasoning is the same as it (presumably) is for there being such a limit in the first place: If you have a 7(or a 5, since someone who reduces their ability score to such degree in point buy may well go all in and pick a race that further penalizes it), the score might as well be 3 for most purposes and you're just not going to use it for anything unless absolutely necessary: A wizard with 7 strength isn't going to be making melee attacks, and if someone grapples you, you're pretty much automatically going to fail any opposed checks. A barbarian with 7 charisma is not going to make social skill checks - he'll let the bard do the talking for him. And a fighter with 7 intelligence is going to take his one skill point per level and ignore anything that requires intelligence based skills(constitution, dexterity and wisdom people aren't going to reduce too much anyway, because every character needs them to a degree - unless someone is planning to become undead). There are some marginal downsides, of course(carrying capacity and ability damage come to mind), but except maybe carrying capacity, they're not something you're really going to worry about (unless you know your GM is going to make sure to engineer situations where your weaknesses come into play).