How to estimate OpEx for self storage facility

[Ross Billington]

Hi all, 

I've been exploring self storage units and am curious as to how to estimate OpEx. It seems like a decent rule of thumb (according to Google and others on here) is that you can expect them to be about $3/sqft. However, after researching competition, both locally and regionally, they are renting them out for about $0.50/sqft. Obviously one of my assumptions is incorrect or I'm missing another vital source of income, but I can't figure it out and was hoping someone on here could shed some light on this.

Thanks!