So, the other day I jokingly twittered (normally, my twitters are dead serious) that a bunch of us new media types should pack up and head to Detroit, where we could all buy mansions for $1000. A friend responded by suggesting some analysis would be necessary before we all up and make the leap. Fair enough.

The idea is this — if enough people of a certain productive potential move to Detroit, then Detroit will begin exercising an attractive force. In response to the growing population of people, supportive infrastructure will grow up. Employers will follow or start-up from among the migrants. Consumption options reflecting migrant taste will appear. And eventually the whole show will become self-sustaining. People who want to be in the industry involved or related industries will move there, employers who want to employ such people will move there, and so on and so forth.

The question is, how many people have to move before growth becomes self-sustaining? The actual answer for Detroit would be unique, since some people in the particular industry already live there. If properly concentrated and supported, the number of necessary migrants might not be all that much. (On the other hand, Detroit has plenty of negative amenities). So let’s take a different case; what if we were starting from scratch? What if I bought a few hundred acres of empty land close, but not too close, to a major urban corridor (say about 200 miles), created a street grid, and started building homes. How many folks would I have to convince, or bribe, to come before the whole thing took off, making me a tidy real estate profit?

Well, that depends. In certain rare cases, a very low population will suffice. Should the first few recruits succeed in creating a famous artist colony, then we could probably get by with only 5,000 or so people. Similarly, should I manage to open up a university of sufficient quality and amenities, then we could probably make it with anywhere from 10,000 to 50,000 people. Same goes given a unique natural attraction. But if we’re hoping to create a true industry center, either tech or media oriented, or both, then the necessary population of professionals is probably much higher — I’d guess at least 100,000.

Why so many? Well, if all we needed were the brains of the operation, then it wouldn’t be that large, but cities are complicated economic ecosystems. The professionals will want to go out to enjoy themselves, which means we’ll need entrepreneurs to open and run bars and restaurants, and a large enough urban population to keep a small but diverse array of establishments open and profitable. People need accountants and banks, doctors and teachers, lawyers and public servants. Cities need plumbers and janitors and sanitation workers and tailors and dry-cleaners and pharmacists and electricians. And if you want more and better options — theaters and music venues, museums, specialty shops, and so on, then you need a bigger population.

Again, you can get by with less under special circumstances — if the migrants are happy with discount options or are interested in simplicity or a DIY ethos (or if you have a bunch of government scientists who never want to leave the lab) — but such characteristics alone won’t generate self-sustaining growth. Most folks are interested in convenience and options, and that means a bigger population. I’m guessing here, but looking at a few metropolitan examples it seems like the sweet spot is a total population between 200,000 and 500,000, most of which will wind up being service professionals supporting the “town industry.” Better planning will likely reduce the number. In a denser environment, more people have access to more places, increasing the likelihood that a larger number of businesses (and a more diverse array of businesses) is able to survive.

So it’s not clear that my plan could work without some serious sponsorship. But still, while our little unsustainable community lasted, we’d all have cheap mansions. And easy access to Canada.