I have not spent a penny of "actual" money, all my purchases have been made using credits earned through surveys. When "browsing" offers, one of the options near the top is to show all the "free" ones. My experience has been best with the ones that indicate "credits available instantly" rather than wait a day or a week for confirmation. Still, they can be hit or miss: not "qualifying" or getting into the survey and getting kicked off or dropped for some reason. Mostly the ones I do are for 75, 150 or 450 (if lucky) credits.
if you don't qualify, try again a few times. be warned though, some say "only available once a day" and they mean it, you could still "complete" a second survey but you will NOT get credits that same day.