Ok, I went back and dredged through some FW Dodge comparatives--initially just out of curiosity on unit size splits, how many 5x, 5x10, 10x10 units most self storage units program. From those I refined my original in-my-head-only estimate numbers--you might could build the building for $175/sf, which would be comparable to Class A lease space prices.
For comparison, non climate-controlled space builds at about $85-125/sf; climate-controlled runs to $150-160/sf (you really need multi-floor to get the unit numbers into as small a campus as possible).
It's a little hard to get good numbers on operating costs--but, most self-storage places are investing in full security cameras, and full access control, and some sort of on-site human presence--so that's a wash, really.
Lets see, 200 units, 25 ultra secure, 50 super secure,, 125 ordinary secure. Let's call it 300sf for 5 units give 12,000sf in units, then 2000sf more for circulation space is 14,000sf facility space, round that to 15,000sf built. That's $3 million to build. 12,000sf of storage at, oh $1.75/sf is $10,500 per month in gross income at 50% rented. (You ought to get $2 for the super secure and $2.50 for ultra secure; but you might not). At, oh 45% overhead cost, that leaves you a net of $5775. Which is not going to work as a $3 million note is [looks online] $12,400 at 4% for 40 years.
So, you'd have to drive the build costs down somehow, and the operating costs (1 employee, 8 hrs, 7 days per week paid $15, costs you $20/hr x 56 hrs is $4480/mo) down, and/or get a lot more in rent.
Some tough sledding on this. I'm using a lot of worst-case to estimate this, too. Climate-controlled space in DFW is running $2-2.50/sf for ordniary security (access control, and you bring your own lock to the steel roller door). Offer true secured, near fire-proof storage, and you ought to be able to get $2.50-3.00/sf, which would be $18 grand a month at 50% rented.