$25 - $75 per month works out to $300 to $900 per year. That kind of $$$ can get you a decent RSC. Two or three years worth of storage fees can get you something really nice (relatively speaking, in terms of reasonable gun storage).
So, I don't see this being attractive to a homeowner. For renters (either apartment or house) who cannot bolt 500# containers to the floor joists or basement floor, it may be a better option. Apartments can generally not be fortified as well as a house since they are usually rented, so security is more of an issue. Space is also at a premium in an apartment.
There may be some legal issues to this too. By being in the business of storing guns, is it considered taking possession? Must the business have some type of FFL? Will the city/county even grant a business permit for this?
Security is a problem too. Will all "members" be allowed to open The Big Door? From there, how easy will it be to break into someone else's locker? How many people are required to open a bank vault door? I've heard weight is not the issue, but do two people need to turn keys, combo locks, etc?
What happens when the neighborhood criminals break in and relieve the facility of its cache of weapons? Will the business be liable to reimburse owners? Will the state file charges of negligence for "providing" guns to illegal possessors?
Run the numbers. How much will rent be and what were the utility costs for the past 12-36 months? How much is insurance? That should establish a baseline for fixed costs. Now look at the size of the place. How many full size long guns can it accomodate? How many full size handguns can it accomodate? Divide it all out and see what your breakeven point is for the price you want to charge. Will you need to store 100 rifles? 200 rifles? 50 handguns? 800 handguns?
Writing a business plan sucks because it gives you a hundred reasons NOT to start your business. However, they are essential because they encourage you to think concretely about the venture. Many ideas sound good on a small scale, but when you toss in all of the government rules and regs, plus insurance, rent, etc, most ideas fall apart.
If you are really serious about exploring this further, PM me. I've had some entrepreneurship coursework as part of MBA program and I'm an accountant by trade and my undergrad degree is in marketing. I'm no business genius either.