Watching a bunch of streams I've seen a lot of beginners in 1v1 (and experienced players) hover over the buy button to see how much money they need to buy their opponent out for the last 5 shares.
Here's a simpler method:
If they have a stock price of $80.00 it will take $800k to buy them out. You have to pay double for the last 5 shares (5x2 = 10). Then just multiply the 10 shares by their stock price and VOILA! ($80 x 10 = $800k buyout price). Essentially just take the first 3 digits of their share price and that tells you how many k dollars it takes to finish them off.
Note that this only works when you already own the first 5 shares of someone and are looking to close out the game by buying the last 5.
Hope this helps. I know this may be obvious to some but if it was really obvious then nobody would be hovering!