This is where your reasoning ultimately fails
Basically, you roughly divide the current market cap of Bitcoin by 2 and get the cost at today's price (since this is what the market cap shows), implicitly assuming that the price will remain the same when you actually start buying 51% of all Bitcoin monetary supply. Nothing could be more false and farther from reality than that. First, not all coins are being traded, I guess, it is somewhere in the range of a few millions (maybe, 3-4 at most), and when you have bought your first million, you will have to spend like 10 times more money to buy the next million of coins (due to prices flying to the moon). Further, you chose to completely ignore my argument that you may never get there at all, no matter how much money you could have since some stake holders may not be going to sell their stakes at any price
First of all, you mentioned a PoS attack from inside.
This would cost exactly 21.678.642.232,83. A PoS attack from the outside, would cost alot more obviously and might be arguably impossible
You simply can't claim that
Mainly, for two reasons. First, there is almost no chance that it would cost exactly that (or any other number based entirely on current market cap, i.e. price). If we talk about an "attack" originating from inside, it might in fact cost next to nothing simply because someone (e.g. a coin creator) might have premined 51% of coins at no cost at all. And once again (this is second), you can't call it an attack at all if the owner (major stake holder) voluntarily chooses to close the whole shebang. In all other cases, it won't be an insider business