My maths is as follows:
A decent return on investment after inflation, fees and taxes is roughly 6%. It might be more or less than this depending on your country and market conditions, but 6% works out to 0.5% a month (if we ignore compound interest), which makes things easy to calculate.
If you had a million bucks making 6% net per annum, you'd rake in $5,000 a month.
So if you make $5,000 a month in IM, your net worth - measured entirely in passive income (which I know ignores many factors) - is about a million bucks.
$500/month = $100,000
$50/month = $10,000
Another way of looking at this is, if you increase your passive income by $20 a month (if you made $80 last month on AdSense and now make $100), that's as if you just added $4,000 to your passive net worth.
So, measured entirely in terms of passive income, a job that allows you to save $4,000 a month is equivalent to working IM and adding $20 a month to your internet empire.
This assumes a very long-term view, though. And because of hyperbolic discounting I suspect that most people would rather work for a month for $4,000 than have $20 a month forever.
Still, if you're planning on making a career out of IM this is an interesting benchmark, I think.