The answer is 845,000. You only need to look at the thousands column, in this instance it's a 5. The 8 and 4 are just window dressing.
Take a look at this example: What is 1,309,452,499 rounded to the nearest thousand? All of the numbers to the left of the 2 are meaningless, no matter how many there are. You should only be concerned about the numbers 2,499. The next step is to decide if you should round up or down. Is the number to the right of the thousands (in this case 2) 500 or above or 499 and below? If it's 500 or above you move up one number making the 2 into a 3 and add three zeros giving you 3,000. If it's 499 or below you keep the 2 and change the other numbers to zeros giving you 2,000. Last thing you do is add back all of the numbers to the left of the 2 and in this case you get 1,309,452,000.