There are several pre-fab T-Hangar manufactures. Erect-A-Tube, Fulfab, R&M Steel, and a few others.
If it's a public agency as someone mentioned, the costs will be a lot higher. Especially if they're using FAA AIP grants; that adds DBE % requirements, Federal Wage Rates, and Buy American clauses that all drive the costs up. If they are self funding, it's extremely difficult to cash flow the hangar; the rents will be so high its nearly impossible. Airports will still build them, because more based aircraft means more fuel sales and FBO fees, so they can afford to lose a little on the hangar lease if the net overall is still positive.
As a private owner, you can't really afford to take a loss, unless you're doing it intentionally to offset your other business profits... For cost cutting, you can be the general contractor, and subcontract each trade separately. Spend the $$ to have the building staff come and erect it will save time/money, too, as they'll put it up in a hurry. Some of the "extra" things that are nice to haves but add a TON of costs are electric doors, insulation, "excessive" electrical outlets, etc. (By excessive, I mean, put anything larger than a single 114v outlet and you're asking for owners to do maintenance & run tools. If that's allowable, your insurance costs will go up. Better you say that's not allowed, and don't provide the power to do it.)
You probably will need an Engineer to handle the site layout & permitting aspect of the project, unless you hire a Design/Build Contractor to just do it all (those are out there, too).
Also, when you're doing your cash flow calcs, don't forget that you'll have 2 storage units on the ends. The airport may rent space for their mower/tractor, or you may be able to rent those out as additional storage. Depending on the T-Hangar size, these can be pretty decent size rooms for an RV or large vehicle as long as you put a large enough door on it. The airport's rules may prevent leasing it out separately as non-aviation use, so you may have to bundle it in with a T-Hangar and then what the renter does with it is not something you'll want to know
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All of this could be misconstrued as advice, but it's not I'm probably not licensed in your state, and probably wouldn't be able to help more than just some general items above. If you're in one of the Southeast or Central states, I can point you to a good Engineering company to handle that side of it and help you really plan it out & come up with real costs. Unfortunately mine wouldn't be able to help as we're restricted from working with private developers 99.999999% of the time.