Tough to replace yourself it due to all the adhesive (see the iFixit teardown for more info, and the iFixit web site has parts and repair instructions as well if you want to go that route, but not sure they have screens for the Air yet).
If you purchased AC+ along with your iPad, it will be $49 for a replacement unit. If you didn't purchase AC+, then an out-of-warranty replacement is $299. That's the "worst case" of what you are looking at. However....
Make a genius bar appointment, and explain the circumstances. They may decide there was some defect with the glass or assembly that caused it to crack, and offer to replace it free. That's unlikely, but may happen depending on how they view the damage.
The most likely outcome, given it's only a couple weeks old, is they will offer to let you purchase AC+ retroactively for $99 and then use one of the $49 accidental damage events available under AC+. That means you'll walk out with a replacement for $149, and you will have the remainder of AC+ (a two-year warranty, and one more $49 accidental damage event).