Calculate rent per square foot and per square meter from monthly rent and property area.
It varies by location and type. US average residential: $1.50-$3.00/sq ft/month. Manhattan: $4-$6+. Commercial office: $2-$8/sq ft/month depending on class. Industrial/warehouse: $0.50-$1.50/sq ft/month.
It lets you compare properties of different sizes fairly. A $3,000/month apartment might seem expensive, but at 2,000 sq ft ($1.50/sq ft) it's better value than a $2,000/month 800 sq ft unit ($2.50/sq ft).