The prices vary hugely. it depends on what day, what match, type of seat etc, and you should look around as there are different companies offering different prices.

If you look on the official site you will see that a lot of tickets have been sold already. Try clicking on the days you are interested in and selecting a seat from the map that will appear. if you can't find seats you could try the Open Ticket Exchange will deals in resold tickets. These are often well above face value - some people buy lots of tickets early just so they can resell them - but I saw some around the $70 mark, which could be of interest.

If you put 'buy US open tennis tickets' in a search engine you will see links to any number of sites claiming to sell tickets at a good price, but you should check very carefully before using such sites because there are a lot of fraudulent ones around.