Screenwriting : Good program for writing plays. by George McMorrow

I am mostly in production and post-production but I do mentor for some schools and also help out with having interns here. I started a new person that has a BA and MA in english and writing and is trying to find find a good career path. The person is really taking well to editing and loves to go on production shoots and is becoming quite proficient at the camera. During our usual BS session, we came up with a brilliant idea for a play. I used to use final draft but my OS update makes my copy unusable. Any suggestions for cheap mac writing program that would include a play format ? I would really like to keep the spark going.