The cheapest traffic source available to an affiliate marketer depends on your approach. You can use free social media platforms to share your links. You can join active Facebook groups and promote your offers. You can answer questions on Quora and add your link naturally. You can write blog posts on Medium or Blogger to attract readers. You can create Pinterest pins to drive traffic to your affiliate links. You can try low-cost platforms like RevContent for targeted traffic. You can participate in niche forums and share your link where appropriate. You can also optimize content for organic search traffic.