system
Given the following post, respond as if you're commenting on Reddit:
human
How is decided what number should be show on the screen of your broker for the price of a financial instrument such as a stock?

I never understood how/what decides the actual price of a financial instrument. I understand that  supply and demand  causes the price comes to a certain point.  

What I'm asking more specifically for is in what way the number is decided for it to be shown up on the screen of your broker. Is it the most recent price a stock has been bought for? If so, why can't you buy the stock for a crazy inflated price? I guess it's something else but that's the thing I'm asking.
gpt
This isn't really an economics question, so you'd probably get better responses in another sub.

But you're basically correct:  the price you typically see when you ask for the price of a stock (or most other securities) is the price at which the last trade was executed.  In liquid markets, this is pretty stable, meaning that someone won't really be able to buy shares at a "crazy inflated price."  Even then, outliers are dealt with how you'd expect them to be.  Market quotes are handled differently by different sources, but no reputable data provider would allow for such misquotes.  This isn't so true for illiquid markets, though, where the "last price" may not be accurate (you see a lot of people get confused by this when trading low-volume stocks and options).

Just remember that the stock market isn't a video game and there are actual people on the other end of all of these trades, so you're not going to just fool the markets by buying shares at absurd prices.  In fact, all you'd be doing is losing money while everyone else trades at a more accurate price (as determined by actual supply and demand).  For example, if MSFT is trading at $300 and someone decides to buy a share for $400, nobody is going to think that MSFT gained $100 per share in value instantaneously.  They'd just assume someone goofed and carry on trading at reasonable levels.

There can be situations where people attempt to trick markets into thinking something is more valuable than it actual is (see: pump and dumps), but that's not really the same thing as just submitting an order with an absurd price in the hopes that people will just assume that that price is accurate.