With $70, you won't be able to get enough traffic to see if the offer is giving you a good ROI. Maybe with $170 if you got a adwords voucher. The problem is that when you work on a ppc campaign you need to run the ad long enough to keep split testing landing pages and ad groups until you find keywords/ad/landing page combinations that convert. I guess you could get real lucky and start making a profit after your first initial setup.