Hi Murray...
It would really depend on what show and when. If it is a show that has been running for some time and you aren't picky about your seating, or you don't particularly care which show you'd like to see, you can wait until the day of show and go to the
TKTS booth at 46th street kiosk. They sell day of show tickets and you can get 3 row orchestra or nose bleed for 1/2 price.
If you have some time and wanted a secure option, then you might try
Broadway.com
Unless you go to the box office in person, most all services charge a fee.
and when you are there, give my regards
