I have said it before, if you are going to use rewards, you have to get it right. Whilst there are arguments about the value and / or damage of rewards in gamified systems, they are still being used and I feel they are of value.
One of the keys to getting the use of rewards right, is understanding their value to the user relative to the actions or activities you want them to undertake. You have to consider the effort it will take. Effort here is: skill, time, physical effort, mental effort etc.
For example, if you offer a user an iPad for posting a single two line comment on your blog, you are setting the reward far too high for the relative effort expected of the user. If you offered an iPad to the person who finds the most bugs in your system, knowing that they will have to spend hours of their own time doing it, that is a better match of reward against relative effort.