My landing page has an email capture form that I quickly change to match different offers. I need not only a low cost advertising channel, but quick execution (unlike adwords).
Depending on a lot of factors you can make Adwords cost efficient. You may not get the volume you want at the margins you want, but go for margins and utilize other channels to reach volume.
Affiliates can be another low cost but effective channel. Affiliate traffic and conversion quality can be great or terrible. You get what you put in and if you manage the affiliates properly you can get good quality conversions at good margins.
I recommend starting with Shareasale. You won't get the highest volume from them, but you'll likely get very good quality if managed properly.
I'm happy to help further if you want to have a call.