If a company has in total 20 shares, selling at the original price with $100 each.
A user (A) buys 10 shares and a user (B) buys 10 shares (which makes half). Then each individual pays with real life money $1000 for their shares (so $2000 in total). The real money should go to some bank account.
If user (A) makes an order, bid for $80 a share. User (B) agreed that price and sells at low price and the stocks go down. (A) pays from his pocket and this money goes directly to (B). (B) earns back 10*80 = $800.
So we have a pool money of $2000 (in some back account) + $800 (in B's pocket) = $2800 worth in real life money exchanged!
(A) now holds 20 shares * $80 = $1600 worth (B) now holds $800 as real life money
The total makes 2800-1600-800=400
So where does the lost money of $400 goes?
On the contrary, if the (A) is willing to buy at the price $120 per share, the stocks goes up. We now have 2000+1200=$3200 of real life money exchanged.
(A) now holds 20 shares * $120 = $2400 worth
(B) now holds $1200 as real life money
The total makes 3200-2400-1200=-400
So where does the virtual extra money of $400 comes from? (because material can't pop out of existence, the real life money is still put somewhere)