Contact Stardock sales/support re: your question about discount pricing on the CD.
For archiving, right click the game in the My Games tab of Impulse and select Archive Application.
Burn it disk, thumb drive, whatever.
In theory, since I've not tried it, you then go to the Impulse menu (top left, above the left and right arrow navigation buttons) and select Restore Archive. You will need to do all of this through Impulse. I also do not know if there are restrictions with moving this archive around between different computers. You will want to test this before relying on it.
Finally, you can go through the Preference menu options and have games auto-archive before updating. This would seem useful if you are about to apply a patch, but don't want to have to reinstall everything from scratch if you don't like the patch content or have some issue with the patching process.