To add on to what ClumsyDude said,
Rush tickets almost always go onsale when the box office opens on the day of the performance (usually 10am), and they will sell them until they run out, or until curtain. So it's really a question of how popular the rush is., which is why people make threads like this one, in order to gage how many people have been showing up. They usually range from around $25 at the lowest, to $40 at the highest. Most of them are are around $30-$37. In addition to the link that ClumsyDude posted, there is also Broadway For Broke People, which I have found to sometimes be more accurate/up to date than Playbill (but not always).
http://www.broadwayforbrokepeople.com/
The most important thing to know is that each show has a different policy, so just be sure to do your research.
You almost never get a choice as to where you sit. I usually ask them anyway, because I've gotten lucky a few times and have been given a choice - pretty rare though.
Updated On: 10/27/15 at 11:19 PM