Although not officially confirmed by Blizzard, but there is string inside of the client that points towards a reward for testing real money store purchases during the Heroes of the Storm alpha test.
Moreover, all real money purchases are currently capped at $325 and they are only accepted via US payment methods.
Heroes Nexus Wrote:
Please be advised that real money purchases will be capped during Technical Alpha. Also, be advised that all gold, progression, and purchases will be deleted from your account in the future during a planned Technical Alpha account wipe.
Following the wipe, purchases made with real money from the store will receive the equivalent amount in credit.
As a thank you for helping us test our store, you will receive a special gift!
Real money purchases can only be transacted with an accepted US payment method in USD. Real money purchases are capped at $325.
At the time of the wipe, purchases made with real money from the store will receive the equivalent credit (e.g. in Battle.net balance or by other means). WARNING: Changing your battle.net balance from USD to another currency will result in the inability to process this credit.
However, this news doesn’t come as a surprise because Blizzard did the same thing with the Hearthstone beta test where players who made a real money purchase would obtain a golden version of the Gelbin Mekkatorque card.
The reward will very likely be cosmetic in Heroes of the Storm as well to not cause game imbalances.