He's the #1 search result for "personal development", which averages 90,500 searches per month from Google. Assuming he receives 60% of those visits, that's 54,300 visits per month. Since it's search traffic, which would be people willing to click and look for info, he might get a CTR of 2%, which is 1,086 clicks per month. The estimated CPC for that term is $1.92, so that is $2,085 per month, or $25,021 per year.
Since that's a rough estimate of $25k/year for one search term, I think $100,000 per year is reasonable.
I've run a site with similar content and good search rankings that gets around 50% of the traffic of Steve's site (according to Compete), and our AdSense revenue was nowhere close to $100k per year. Even though Steve was using AdSense more aggressively, it makes me wonder how he was able to earn so much. The average earnings per click for us was only 15 cents or so.
If you are writing constant posts on personal finances: remortgag, consolidating, getting a financial advisor etc. & the ads that get posted are reasonably relevant (very dependant on the industry) & the readers are 'get a cup o coffee & read' type (digg/stumble/slashdot are reportadly click happy 15 second visitors), then you can get a good amount.
Its important to think about this from the advertisers perspective. The industries where click costs are high are where a large number of clicks can become substantial sales. Services are a biggie: everywhere from mortgage consultants to plumbers.
Having said that, Slashdot readers are more likely than average to use ad-blocking software.