If you're on IOS 7.1 I wouldn't recommend it. If you're on 7.0.x then
http://evasi0n.com/ download the program, plug in your iDevice, open the program and click jailbreak. Once you've done that go to Cydia and go to manage>sources and add the repo
http://repo.biteyourapple.net
Once you've done this, go to the search bar 'LocalIAPSStore' and download it. Following this, go into settings and scroll to the bottom, there you'll see 'LocalIAPSStore', click it and enable it. Now, when you press purchase on most in game purchases, once prompted for your Apple user ID and password, click cancel and the item will be purchased