It depends on many factors; size of the unit, area, outdoor, climate control.
Here are the average prices:
A 5x5 foot storage unit usually runs about $40 -$75 per month.
10x15 $75 -$140 per month or $115 -$150 for a climate controlled space.
10x20 unit ranges from $95 -$155 per month or $170 -$180 for climate controlled.
Prices are higher in dense urban areas than in rural areas.