According to the Microsoft store website, if you pre-order Grand Theft Auto V you will receive $20 credit towards XBL purchases!
The post states it would be 1600 Microsoft points (which is the equivalent to $20), but with the recent change from points to actual currency, this has been amended on the site as well. It still states it as a 1600 point redemption with the additional information displayed beside the asterisk.
This offer is valid until September 16th, one day before the release of GTA V. This is a great value! Although if you want the game by the midnight release you’re probably out of luck reserving it in this manner.