Hello I'm ready to buy a 2003 Toyota Matrix it has 240,000 miles how much longer will I get out of it mileage all many more mileage will it go? Another 200,000 would it go to be 440,000 miles?

Unfortunately your question can not be answered - YET - only time will tell. It depends on maintenance history thus far .. It would be possible but I think 440k is wishful thinking. Good luck!