How much does it cost for a single incident of support?

From the link in the support document you came from:

http://www.apple.com/support/products/pay-per-incident.html

$29 USD

To purchase pay-per-incident support, call
1-800-APL-CARE (1-800-275-2273).

For Apple Products

Purchase a single incident of support for help troubleshooting Apple products such as AirPort devices, Apple TV, iPad, iPhone, iPod, or Mac computers; Apple operating systems such as iOS, OS X, or OS X Server (GUI only); Apple-branded consumer apps, such as FaceTime, Find My iPhone, iLife, iTunes, or iWork; or Apple-branded pro apps such as Final Cut Pro and Logic Pro.

Depends. There's no set amount.