The average national storage unit costs are: 1  $40-$50 per month for a 5-by-5-foot unit. 2  $75-$140 per month for a 10-by-15-foot unit. 3  $115-$150 per month for a climate-controlled 10-by-15-foot unit.  $95-$155 per month for a 10-by-20-foot 1  unit. $170-$180 per month for a climate-controlled 10-by-20-foot unit.  $225 per month for a 20-by-20-foot unit.