I've used a "regular" storage facility that had open areas where they rented a space to park, that's where my RV was broken into. About 400 customers had rentals, who knows how many more had the code to the gate?
I've moved to a RV and boat storage space that has mostly RV-size sheds and a few parking spaces in the open. Each customer has a key to the gate. All are RV or boat owners. In nine years, no break-ins.
What it will cost depends on where it is, value of land, zoning, insurance costs for owner. In my small town, I can park in a shed for about $35 a month. For a similar facility within 15 miles of a city of 500,000 the cost is $120 to $150 a month. I know an indoor storage warehouse in Chicago that charges $400 a month for a RV-size ground-floor drive-in/drive-out. In my town I can rent an 800 sq ft home for that.
All part of "cost of ownership" as discussed from your other posts. Storage and insurance costs are very much location dependent. If you really live in the wrong place, easily into five figures annually. There are places where insurance and garaging run $10,000 a year for a car, and most people don't own cars, it is less expensive to rent or hire livery.