Earlier this year, Detroit’s condominium market had 11 months of inventory. Now, it’s down to 7.6 months of inventory, ranging from downtown to areas east, north and west. That promises a healthy supply and demand, albeit probably short-lived. The rising costs of building materials, as much as 50 percent, are driving construction costs up and could threaten to cripple condo development projects. Increases in price may drive future buyers away. Empty nesters and older buyers are gravitating toward luxury condominium purchases, while first-timers or younger buyers are deciding to purchase single-family homes after seeing the high prices of condos.