I started as something similar to Peepcode in December 2011 and was charging around $12 per exercise but I made it free after 8 months during which I sold around 6 exercises (to 3 people). However, I don't want to go back to this after making it free. Being free makes it available to a lot of people and it's something I won't change now (I actually wish I had start with free exercises).
I offer commercial license to use the exercises to run training with not much success so far (even if it's pretty cheap $990). I have around 130 visitors/day on average and up to 4000 when an exercise is released (the average value is 2500). I tweeted about sponsoring but I didn't get any reply so far.
And now I'd like to see if HN has a better idea or what I'm doing wrong... or maybe it's just not something that can be done and I should just accept to do it for the fun of it.