This survey site takes a while to accumulate enough points to redeem ($50 Paypal reward = 40000 points) most surveys are worth between 700 - 1500 points. The surveys are always interesting and don't take too long to complete. You can also get rewards to use at Amazon, iTunes, Wotif, Flight Centre. This site also gives you the ability to donate your points to save a number of square meters of rainforest.

Approximate cost: $Free