If you want to control the level of security for each step of the purchasing process, go to: Settings --> General --> Restrictions.
Once in the Restrictions menu, you can enable/disable restrictions there by entering your password.
After restrictions are enabled, search for "In-App Purchases", where you can completely disable in-app purchases (turn the settings to OFF), or manage the frequency of password enquiries by using the "Require Password" button. If you want to keep the ability to make in-app purchases, yet want to add additional level of security, you can set the device to require your password every time you make a purchase (select "Immediately") or after a set amount of time (e.g. every fifteen minutes).