The pricing behavior of a modern economy is entirely dictated by the component prices of four things: Energy, Real Estate, Food, and Water. There's some interplay in how the pricing of one of these impacts the other (e.g. expensive energy makes transporting food more expensive, but expensive food makes harvesting energy more expensive). But there's nothing more "atomic" than these four things; the price of everything else is overwhelmed by price movements of Energy, Real Estate, Food, and Water.

(in a competitive market, is the asterisk on this. If a market is not competitive, then Greed can be thought of as a 5th atomic economic input).

The biggest challenge of the 21st century is: we aren't discovering much more of any of these things. The second derivative of "how many of these things are available on the market" is basically 0. Rights have been sold to everything in the ground; farmers know exactly how many cattle they'll have three years out; there's no surprises left. Companies need to show revenue growth, and Jerome wants 2% inflation, not 0%, not 6%. So, the price of these things can only go up; nothing is forcing them back down.

The situation for Energy, Food, and Water isn't great, but they all have a pretty constant cost to their production; there's some sources of energy that are harder to get at, I've always heard fracking is one of these, but by-and-large they still have economics of scale on their side, once you adjust for inflation gas was $3.14 in 1975 and its $3.21 now, 50 years later. Its a similar story with food. Water has probably gotten cheaper, actually, but that's a rounding error.

Real Estate is the opposite. We're making more people. We aren't discovering more land. Critically: We can increase our effective utilization of each square mile of land, but doing so raises the cost of each unit. Its cheap to just throw a homestead on a plot in the middle of nowhere, but once you put 200 people into an apartment building the same size you need to start thinking about parking, transportation, plumbing, electricity, crime, internet, it gets more and more expensive per-person as density goes up. This is part of the fallacy of thinking that the whole solution is density: Replacing a single family home with a 50 unit apartment complex usually results in an increase in cost per square foot, not a decrease.

The other part is highlighted in Harris' plan to give first time homebuyers $10,000 toward a down payment. The reason why housing is expensive is not strictly density (read: supply); its also in demand. Demand does not decrease because you built more units. Due to induced demand, it oftentimes will increase, because those units might be mixed use, foot-traffic draws cool businesses, people want to live there, and thus your big plan to reduce the cost of housing by building more units actually just increased it.

If you were the commissioner of some county with a growing population who wanted to reduce the cost of housing in the county, and you were also God and knew exactly how many people were going to move to the county in the next year, and you added precisely that number of units: The cost of housing will still go up. If you add more than exactly the needed number of units, the cost of housing might stagnate or go down, but its likely the vacancy rates will cause some level of financial strain on the property developers, and it might be hard to sustain such development; and in N years the cost will continue to rise.

Developed Urban areas cannot escape this curse. Housing costs will always want to rise at a rate higher than inflation, over a long enough period of time. This shouldn't stop cities from increasing density, because what other option do they have, and it might be the difference between 4% and 8%. Underdeveloped cities (e.g. Austin TX), suburban, and rural areas in the United States can still underrun inflation, however, but shouldn't rush to significantly increase density more than demand on the area can support.

The idea that any given county with a growing population can meaningfully and durably reduce the cost of housing within their borders is, mostly, a fallacy in the United States. The only way this can happen is in an environment with deflationary monetary policy, and the United States is extremely allergic to this.