Not all in-app purchases can be re-downloaded for free - from http://support.apple.com/kb/HT4009 :
If you lose your In-App Purchases because apps were accidentally deleted, or if you had to restore your device without a backup, you may be able to download some of your in app purchases again for free. Not all In-App Purchases are eligible for free download again.
- Replenishable In-App Purchases and subscriptions cannot be downloaded again for free, and must be repurchased. Examples include but are not limited to: extra ammo, additional health, extra experience points, or other one-time services.
- Non-replenishable In-App Purchases and auto-renewing subscriptions can be downloaded again for free. Examples include: bonus game levels, a map/city guide, a daily news subscription, or other recurring services.
- To download an In-App Purchase again, you must download it from within the installed app using the same iTunes Store account name for the original In-App Purchase.
You can try contacting iTunes support (these are user-to-user forums) and see if they will refund or credit you, but if they are the non-redownloadable type then they might not : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption