That is the very article I was thinking of Old Man.
It is one of my favorites as well. I have saved it to my hard drive and break it out anytime I get to designing a fantasy world. One of my pet peeves in many fantasy worlds is just how many sentient people are around - people forget how much lower the population density was in medieval times.
One of my favorite sections is the distribution of types of professions found in a city as well. It is based on a single data point, but that is better than nothing.
As for Dagorhir's concern that my approximation overestimates the cost of living, My estimate isn't just for the price of housing. I use the cost of a nights stay in an inn as I rough guide for the cost of living a day (which includes housing, food, and everyday expenses and consumables). So while an inn does need to markup the cost of housing when it "resells" it, I guestimate the markup roughly equals the cost of ingredients for home cooked meals, fuel costs, thread and wool costs, thatch costs, and other everyday consumables. In medieval times clothing (a highly worked product) was a borderline capital expense, so I divide the costs of a suit of clothing or two over the days of the year and add it to the cost of an inn. I also know that even desperate people will "waste" some effort and perhaps even money for basic entertainment in the long run, so add a small amount (usually just enough to round things out to a convenient number) as a bit of a "fudge" factor.
I have checked it against some "real" numbers as best as I can for a few fantasy systems and it hasn't proven completely unreasonable so far. So I continue to use it for a rough guess.
Another assumption I make with this system is that most of the "income" or "cost of living" isn't going to be in the form of money for much of the fantasy setting's population. The actual "wage" someone earns is likely to be significantly less than this number with the difference made up in barter and self-made/grown goods. This number is just the cash equivalent of those incomes.
It also seems to work OK for Sci-fi games as well (I've used this to convert credits to $/euros for Harp Sci-Fi as well and again reality checks against specific items seemed pretty close).
But as is almost always the case, someone else's results may differ.
-Pyrotech