Be honest. Talk about what you like and don’t like. Be fair and build trust. It will serve you well later. For examples, check out my review posts about Elite Blog Academy and Self Publishing 101. I get emails frequently from people who tell me they decided to purchase one of those courses through my affiliate link because it was the most balanced review they found.
Outbrain is not necessarily the network of choice, it was just an example. As for tracking the ROI usually you'll want to track campaigns in Google analytics and track clicks on affiliate links as event and average value/email or you'll want to duplicate the post and run paid traffic to the duplicate so you know exactly what the conversions are as you can put them in separate email lists etc too.
Promote products at various price points. Even the little products (like Amazon ebooks) add up. If there is a truly useful product on the pricier side, it can still be worth the promotion even if only a few people buy it. If you’ve used a product of exceptional quality and it’s a good investment, or if it’s a product that’s unique, specialized or one-of-a-kind, go for it.