Sorry, but I'm only interested in an answer to the OP. I'd rather get no posts than posts that say "You're doing it wrong", thank you.
The answer to the OP is that it can't be done, just as the answer to "(-1)^(1/2) = " is that it's imaginary. I can't help you if you're asking for the impossible, and I don't particularly care what you want. You ask a question about game balance on a game balance forum, you're going to get responses discussing the balance implications of your question. You can clarify your question, revise it in light of your responses, and otherwise attempt to guide discussion toward the end you're actually attempting to reach, but I'm giving you exactly what you asked for. It's all well and good to request that people respect your wishes as OP, but context is everything, and this board is not one where you can reasonably expect people to ignore an interesting idea (which this certainly is, regardless of its merits) if their opinion on it differs from yours.
In short, if you aren't prepared to hear answers you don't like, don't ask questions.
What you
can do is list a few more constraints on what the item actually does - if it flat-out doubles your ability score, as the OP indicates, it's entirely impossible to price fairly because it offers an unprecedented force multiplier that is as inherently unfair as a circle is round. This is because it will multiply all of the bonuses you can pay more GP for, leading to a truly monstrous score, making it an additional benefit rather than an alternative.
If it only multiplies your
base ability score, and does it in the form of an enhancement bonus, then it's just a bad idea that adds nothing meaningful to the game, but it can be priced "fairly", which is what you're interested in, so here goes.
A fair price would be the price at which 15% of WBL equals a character's base ability score squared, times 1000. The character in question should be one who began with an 18 in the score, a +2 racial bonus, and has put every point gained from leveling up into that ability score. I don't have the ELH right now, and actually calculating this price is more effort than I want to invest, but you're free to. At every level below this, the item is too good, and at every level above, it's too bad, but at this exact level it's fair.