Until a few years ago when an unscrupulous partner ran it into the ground, I had a network of about 8500 domains, 2200 of which were developed into mini sites. Those sites were making $4 to $6 per day on average, with 20% making less than a dollar and 10% making more than $3.
Suppose it costs you $10 to run a domain for one year,
$10 / 365 days = $0.02 per day
As you can see, it's not as hard as it seems if you have good keyword domains. Unfortunately, it can be a double edged sword.