Multi-families tend to follow the overall residential market. During a downturn, they will tend to lose about 20% of their value (and most residential properties tend to lose that much peak to trough). There are many on this board though who say that this peak to trough will be more this time, so take all this with a grain of salt.
Commercial property (pure commercial properties, those that rely on business rent) will go down about 30% peak to trough.
Rents can increase or decrease in a downturn (they tend to increase in a downturn). If rents do go down, they usually go down 10% (absent some major catastrophe).
All this information I learned from my real estate class. Other investors may have other perspectives.
