If you are doing a giveaway, and it represents a cost to your business, it is important that the cost of your giveaway is of a lesser cost than the value provided by the success of the contest.

There isn't a one size fits all equation for figuring this out, so your goal assumptions have to be accurate. If you are testing the waters, it is always easiest to giveaway product, services, or have sponsored giveaways that represent limited overhead cost to your business rather than a prize you have to pay for.