Price needs to accomplish two things - ensure it enters the game at the correct level, and ensure that it uses an appropriate fraction of a character's resources at that level and all future levels. If the item flat-out doubles your score, then, at best, 2.5 million accomplishes only the first. As you rise in level, it becomes a vanishingly small part of your wealth, while contributing a consistent amount to your abilities.
So, yeah, 2.5 million is an answer in the same way that 5 is an answer to y = x^2. You could pick literally any other number and it would work just as well.
If you meant something else than what you put in the OP, such as the suggestion I keep making, then it's possibly a fair price, but you'd need to actually run some numbers, which should be fairly straightforward for somebody who actually has access to their book collection (I provided the two functions you need to find the intersection of in an earlier post).
Again, yes, you didn't provide enough information. Concise is not the same thing as open-ended. You provided a specific concept within the rules, and asked for the resources it should consume in relation to other, similar, concepts. "An item that doubles an ability score" is a concept the game rules can already parse, not an overarching category that needs further specification in order to work. You could see an item printed that says, "The wearer of this belt doubles his Strength score for as long as he wears it.", and it's a different item from the one I keep talking about. If you didn't mean to ask about that particular kind of item, you need to specify that you're referring to a category, or you're going to get consistent misunderstandings such as the one that has so long plagued this very thread, where people answer the question you actually asked.
Communicating badly is not an effective strategy for getting useful answers. I don't know how I can be any clearer.