oh no, I'm sorry to say but building your own is 100% cheaper
Because you *will* have to fix up any cheap prefab to be predator proof and have adequate ventilation. You will. Or, you will have dead or sick chickens.
Converting a shed is probably one of the best ways (and most expensive) for those not handy.
Prebuilts are either exorbitantly expensive and still not really advertised for as many chickens as they claim they can hold - I'm looking at you, Carolina Coops. They are WAY OVERPRICED and you can build one yourself (or hell, download a PDF of plans and HIRE someone to build one for you) for WAY LESS than what they charge.
Or, you buy one of them from Tractor Supply or Amazon for 'cheap' and yeah, they're CHEAP. Unsafe. Dangerously unventilated. So you need to spend more to make them safe and ventilated.
That is why people here, who have *spent the time and experience learning already* that building IS the cheapest, best way to actually get what is really needed for happy and healthy chickens.
Sometimes - you can find a good prebuilt that someone has built on Facebook or Craigslist that is actually pretty good and not too pricey. You will still need to fork out $$ to make it predator proof on site at your location, but this option is rare.