The very first affiliate program I reviewed, paid an average of ten percent commissions on each product sale my site generated. The products (mostly books) averaged around $15 so my share would be about a buck and a half per sale. I figured if I could get one sale out of every 35 visitors I sent to the site, that would be a decent conversion rate (better than average, actually). After doing a little math, I concluded that I would earn about $45 for every 1000 visitors I sent to the site.
I’m always shocked by the amount of content in each blog post on this site… No matter how many times I search the web, I find this site in my top searches with QUANTIFIABLE content. It’s valuable. Great stuff you provide. And SO MUCH. It’s one of my favorite sites to review and read, seriously. I often link back, just so I can remember where I found the information. Wonderful information to share! Thanks for being prolific.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.