I don't know the specific offer you are talking about, but I know the type. In most cases what you sign up for are clubs, memberships or trials of some sort. And you have to give them a credit card to pay. More often than not, the amount you have to purchase winds up pretty close to the "Award / Prize".
If you do decide to try, make sure you read all of the terms, policies, etc not only of the survey site page, but also what you are signing up for. Many have recurring monthly fees that you need to cancel before the next billing cycle if you don't want to keep the service.