The 10X Proof compares a regular wood 4-unit building with a concrete 10-unit building. Both require a similar investment and have the same total rental space. The concrete building makes about $41,100 in yearly profit, while the wood building makes only about $3,200—almost 13 times more.

The study also says concrete buildings can cut costs by more than 20% because they need fewer repairs, have no shingle roof replacements, and may have lower insurance costs. The main point is that rent is set by the market, but building materials affect how much money you keep.