(1) How much does it cost ?
You can download it for free at http://www.nero.com/en/631927526314911.html
(2) Can I distribute the software that I made with NeroSDK as a part of commercial software ?
Yes, you can. But your software needs on every computer it should work on, a Nero version with a valid license installed.
For details on the license please also read the file NeroSDK_license.txt that is contained in the NeroSDK package.
I made a multimedia application that needs a functionality to burn VCD / SVCD. So I just wondering if I can use NeroSDK for this purpose.
Yes, the NeroAPI can burn VCD/SVCD. So you can use it for this.