Answer:
This is going to vary but storage company & location. Best thing to do is call several local storage places and get quotes from them.
I recently compared square footage prices for secured, climate-controlled warehouse storage in vaults v. self-storage with the same components. For a fair comparison, one must consider the actual cost of the space, utilities, security, environmental controls (mostly humidity) and personal property insurance.
Self-storage came to $1.44 per square foot. Warehouse storage, $1.66 before overhead and profit. O&P adds approximately $.97 to the cost for warehouse storage, same factors.
The biggest issue is insurance: coverage, exclusions, ACV or Replacement Cost, ad nauseam. I would suggest a face-to-face with your agent for an explanation, write up a memo to him summarizing the meeting and the information provided and mail it to him and to yourself, certified mail. File your copy unopened for future reference if needed.
Prices are higher in dense urban areas than in rural areas, so it's difficult to provide a definitive answer. Storage can be rented for as little as $10 a month, or as much as hundreds of dollars per month, depending on location, size, and amenities (like climate control).
Did you know that in the U.S. there are more self storage facilities than Starbucks stores?! So odds are there are more than a few facilities near you. Many specialty search websites enable you to compare the costs and features of self storage facilities without picking up the phone. Try StorageFront.com, Spare Foot, or US Storage Search.