The only underlying metric for that value is demand for the coin.

If the population is falling then the demand for housing will tend to fall unless housing units are being torn down.Other metrics?

Multi Family Housing in NYC metrics are:Cap Rate : Net Operating Income/PriceRent Multiple: Price/Rental IncomeAverage Rent Per Apt Per month : Total Rents/Total Apts/12Price per Square foot: Price / BUILDING SF ( not lot SF)Neighborhood LocationThese are your "pitch" metrics.

And to add to the great advice above about diversification is that use "Capitalization Rate" as a metric to level the playing field.  

Don't get me wrong, people are definitely investing there successfully, but as far as comparing it to other markets goes, it doesn't perform very well with the metrics most people are looking for: growth market with a strong economy, diverse job market, affordability (residents make enough to be able to buy property, which drives property values up), good schools and an increasing population.

When I started out I did research on the basic metrics to figure out what the h**l I was doing.

The biggest metric to track is net worth on your balance sheet as this will help you start to "snowball" your investments, make you more attractive as you begin to scale your investments and ultimately the determination of your wealth which has the most impact on your lifestyle and impact you can have in your life. 

I'm still using this method to underwrite deals (link to my recent blog post on metrics). 

K.I.S.S.To date, I still still use this metric.

Quote from @Greg Parker: In a number crunch of 300 cities using 10 metrics ranging from home prices to tax rates to vacancy levels, WalletHub determined that Montgomery, Alabama, was the nation’s most affordable city for homebuyers.